It's the loosening of the Pattern, which also caused a lot of other things. Near Perrin it was ripples, in other places reality was temporarily altered, like corridors vanishing in TV and Caemlyn.

With Mat it was a village from the past suddenly layered over the present.

The ripples would be coherent with a massive use of balefire, which would burn suddenly many threads and force the Wheel to quickly re ajust "reality" so the Pattern doesn't unravel.

However it's definitely not linked to Natrin's Barrow. Even with the Choedan Kal the balefire can't reach very far back in time. Hours, days at most.

There are other explanations too. Balefire is one known cause, but the effect is still a sudden re adjustement of a loosened Pattern. Another cause would of course be chaos. Chaos is by nature and definition irrational and uncontrollable. It's intuitively obvious that the more chaos there is, the less the Wheel is able to keep things on course.

So far, that we can tell (the sample isn't big enough to prove anything, the phenomenons are different whether they happened in big cities like Tar Valon/Caemlyn or in more isolated locations. For all the ongoing chaos in Tar Valon and Caemlyn, both under siege and caught in civil wars, those cities remained to an extent organized, their reality held by hundreds of thousands people. There at first, we have seen ghosts appearing briefly, and small and temporary alterations of reality, like corridors being "re organized". It's only after Egwene's capture, when the Tower and Rebellion well and truly fell into chaos, which also coincided with Rand's sudden increased in madness/darkness, that we've seen much bigger alterations of reality and alterations of the physical laws.. melting stones and metal, permanent changes to the Tower layout etc. In Caemlyn, the worse we've seen was all during the civil war. Things seems to have quieted down a bit, or at least stopped worsening, after Elayne won the throne and restored order.

The more "impressive" alterations we've seen, we've seen happening in the middle of nowhere or in places where chaos ruled. Eg: Arad Doman and in Altara outside the areas held by the Seanchan. Around Mat in the countryside it was far more than a ghost, it was a whole village of them, complete with the village itself, the past of that area suddenly overlayered its present. There was the Hinderstap stuff as well.

Perrin was also in a countryside left to anarchy, with the Shaido but also within his camp, where only his presence held all the factions and conflicting goals there together. But Perrin was also terribly focussed, all his will and actions aimed at one clear goal. His focus/obsession was always driven by the Wheel, and his nature as ta'veren is what held his followers together. That Perrin's obsession was resulting from a pull of the Wheel got confirmed later, when as soon as Faile was rescued, the obsession completely vanished and didn't result in any strong feeling or fulfillment or elation or even relief. Perrin just didn't feel obsessed with Faile anymore.

I tend to think it's Perrin and his focus who caused the ripples. Why they had to occur was because of the ongoing chaos (the DO's touch, and also the consequences of what tons of people have done, including Forsaken like Sammael with the Shaido, and whoever drove Masema to his obsessive madness), but I think the ripples are the result of the Wheel barely managing to hold the Pattern against bubble of evil type of alterations of reality caused by the chaos (ie: they come from Shai'tan, but chaos seems to losen the pattern and make their effects worse). Mat was using chaos to his advantage - hiding in between pockets of chaos so to speak, Rand back then was still creating chaos around him trying the wrong way to bring order back (the one time we saw ripples around Rand was after he balefire a whole palace and the Wheel had to re adjust the Pattern and prevent it from unravelling), but Perrin always fought chaos, aiming all the chaotic factions around him at one single goal, never allowing anyone to stray from the goal very long, even when he was attracting to him some of the few remaining key elements, like Tylee's army and Galad and Asunaway's respective forces. No doubt his ta'veren nature worked overtime to hold it all together.

The ripple occured a few moments before Faile received the Binder that turned into the key to everything, as it's what set in motion at exactly the right time Galina's trap that kept Faile and key people (most of all, Morgase - the only one who could channel to move that little flag that brought their rescuers) out of the battle of Malden. I think the ripple was probably meant to avert something that would have made that course of events impossible, eg: a bubble of evil was about to erupt and was risking to kill Taril before he got to Faile with the stolen binder, or kill Faile herself, or Morgase, or Galina etc. As far as we can tell, that re adjustement of reality affected Perrin's camp, and Faile's surroundings.

Galina didn't care one bit about Faile, she cared only to get her hands on the binder. She set her trap to kill Faile and the others solely to make sure those who knew about her escape could be tortured and forced to talk by Sevanna.

The other rippling effect happens during the last meeting on the even of Malden, right after Perrin gets news that Galad's WC are 50 miles from there, and that more Shaido septs are approaching Malden. Then there are three ripples, and when the others start wondering what is happening, Perrin re focusses them by saying "all that matters is Faile".

I pretty much dismiss the possibility that those ripples were balescreams, when the Pattern re adjusted itself/held itself together with difficulty. Those ripples were localized (Perrin's camp and Malden, it doesn't appear to have been felt by the Whitecloaks). It would have to be a fairly massive use of balefire, and to center around Perrin and Faile like this, it would have to be the removal from the Pattern of something that had affected directly Perrin's camp/the Shaido camp, and in the last hours or days. There's simply no valid candidate for something large enough to have caused balescreams and which had affected Perrin's storyline recently (it's not the DH, that happened too far in time and didn't concern the Shaido camp or the town of Malden in any way).