I always figured that it was one of the Forsaken who added some traps of their own to Callandor, hoping to hurt/kill Rand when he eventually went to fetch it.

One of the reasons I suspect this is the fact that Rand sent Narishma there in the first place. Why would Rand not have gone himself? With traveling he could have done it near instantly. What would have happened if he had? Rand knew exactly where his traps were, so it seems likely that he would not have been all that cautious in removing those traps, why would he since he knew them so well. Odds are that Rand WOULD have triggered the forsaken traps in his haste and overconfidence.

Narishma on the other hand was likely extremely cautious in his work, since he knew he was dealing with lethal traps made by someone else. Even with Rand's teachings, Narishma would not have had the intimate knowledge of the traps Rand had and would have been very carefull, carefull enough infact to notice the added traps and avoid being killed.

I wonder even if maybe the whole reason Narishma went instead of Rand was a ta'veren manipulation to keep Rand alive by sending someone more likely to notice the forsaken traps and successfully avoid them.

As for why the added Forsaken trap was simplistic enough that even the relatively untrained (in comparison) Narishma was able to avoid it, well it makes sense that the forsaken would have to be extremely subtle in their added traps to avoid triggering Rands pre-existing traps and getting fried. This likely meant that the traps were crude by AoL standards. Though still capable of killing Rand if he tried to remove his own traps in a rush without showing the extra-caution that Narishma did.

Ultimately though thats just one of many ideas. I don't know if it has been confirmed, or ever will be. I suppose this is something someone might ask BS. Since it is likely that this won't play any more significance in the future, we might be able to get a solid answer.