This may be too obvious for some of you guys, but obvious isn't always bad.

Moridin was very vocal with Graendal about disrupting any chance of an alliance between Rand and the Seanchan. I think we will see this culminate in tragic happenings around Rand such as the death or compulsion of Bashere. I think we will probably also see Graendal die in this book. It will also be interested to see how A'rangar plays into this picture.

The shadow also tried to disrupt any possible truce between Mat and Tuon. They went to great lengths to send an army to hunt and kill Tuon in KoD. This army was wiped out by the Band of the Red Hand.

The shadow also saw an alliance begin to form between Perrin and Tylee. The shadow was probably very concerned about a ta'veren's effect on a newly raised to blood Seanchan general. Seeing that the shadow didn't have any human armies around (they wasted that on Mat) they sent the Trollocs down to kill Tylee.
It seemed very apparent in this prologue that the Trollocs were assigned to assasinate Tylee above all things. The shadow wants to prevent alliances with the Seanchan above all things.

Therefore, after second consideration, it seems fairly obvious that the Trollocs were sent by either Moridin directly or by Lanfear and Moghedien through his order.

I think it is quite possible that one of Perrin's major roles will be to complete an alliance with Tylee's forces. It would seem that the Seanchan is a 4-headed dragon that can not be tackled by any one major character alone. It is going to need to be broken up into several large assignments:

1. Rand and his army against the Seanchan army in Arad Doman.

2. Mat and his marriage to Tuon

3. Egwene and her struggle with the Seanchan at the White Tower

4. Perrin and connections with Tylee.

I think it only makes sense that the shadow would want Tylee assasinated.