It wasn't "an army" as far as we know, but a band of indeterminate size. Not big enough to deal with Tylee's forces that must have looked battered after so long campaigning and walking, but was stronger than the Trollocs expected. After the initial surprise, Tylee's forces dealt with them.

The fact they attacked Tylee speaks against the idea they had another target, especially a covert mission. They fell on Tylee's forces, it's not like they did bump into them (it didn't look that way, anyway).

To have been on a "mission", it would have required Myrddraal with them. If there were Myddraal, they would not have let the Trollocs attack Tylee if they were supposed to sneak all the way to Ebou Dar secretely. It's also very strange that the Myddraal would have remained so close to the main road (where there are many soldiers and many villages and towns, it's one after another in this area, from Mat and Tuon's KOD travels).

This speaks of a band without Myrddraal to lead, and this in turn speak of a lost group without a specific aim, seizing opportunities the way Trollocs left to their own devices do. If that's the case, and in the light that we know only very recently Shadowspawn were forbidden to go out of the Blight, then the group most likely belonged to the forces sent to attack Rand in Tear, and got lost on the way. We happen to know there was a Waygate north of that location, which Mat passed on his way north in KOD.

Some believe their mission was to attack Tylee, but why? To prevent her from reporting about Malden and Perrin to Ebou Dar? As the Shadow would know about Seanchan, it's way too late for that: Tylee had a raken and sent her reports, and to'raken were sent to fetch her prisoners. Much easier to assassinate her and her officers than try to destroy her whole army if that was the goal, anyway. Pretty pointless to send a group of Trollocs to deal with an army with damane, too. Obviously, the Trollocs had little idea what they really got into attacking Tylee (which speaks against having Myrddraal to lead them, again).

There are others who believed this was a band (out of many) of Trollocs sent to find Tuon. That's not plausible. Semirhage didn't seem to care much one way or the other and already had Suroth doing that search, Moridin had just reminded the Forsaken at that time that Shadowspawn were ordered to stay in the Blight, and finally, that's not a job for groups of Trollocs. Ravens, Draghkars, greymen and even darkhounds, but not Trolloc bands. (not that I believe this is what the DH in WH were after, I still think they are after Fain who had vanished and could have been there but was actually further east, in Far Madding. I'm by no mean convinced, but I have an idea for what Fain may have done after vanishing out of Far Madding and where he is now.).