From the mechanical point of view, I've observed that if multiple non-boss AoEs initiate at once and they were in the bad, sometimes the trusts will run from one into the other as their AI likely doesn't account for the second (or more) AoEs. AI during bosses is usually better, since I assume moves triggering sends the trusts special instructions unique to the encounter, but sometimes you'll get the derp there, too. Probably what makes trash pulls suck is that if they do move, they'll stop acting until the offending AoE ends, which has led to some bad healer moments.