They didn't appear to be targeting anyone in particular but simply be predators in the country side looking for preys, that spotted Tylee's army that looked tired enough to risk attacking them by surprise.

If they had a target (fourty miles south), then they had to have Myddraal to drive them because Trollocs on their own won't stay on target. If they had Myrddraal, then the Myrddraal would not have let them get anywhere close to the main road, 10 leagues to their target.

It would have been stupid of the Shadow to send Trollocs to Ebou Dar from the north. This area is crawling with Seanchan soldiers, all along the Ebou Dar/Lugard Road. It's their main line north. The Trollocs would not have survived long when they approached the city around which there are many military camps (the officers are housed in the inns, their armies are stationned all around, the Tinkers are in between). These Trollocs got lucky. If not for the fact Suroth sent at the beginning of KOD most of the rakens out of Altara (an order recently rescinded by Galgan/Yulan at the time, they would have been spotted by patrols long before they got anywhere close to the city.

The most simple theory about these Trollocs, given the timing of all this, their location and the comments by Moridin in KOD that Shadowspawn are forbidden to leave the Blight is quite simply that this band got lost getting to the attack against Rand in KOD and exited by a Waygate in Altara, most likely the one at Londaren Cor, which was leagues north of the attack on Tylee. They were making their way south, likely because they found more and more villages to attack in that direction (a lot of the population is concentrated in the 100 miles around Ebou Dar).

If the Shadow gave a damn about the Tinkers, they would have been exterminated long ago, not now at the last minute. Nothing has yet happened to make the Shadow see the Tinkers suddenly as a potential threat.