Some enemies like to run away briefly, before coming back. Others might get feared, and run off screen.
Of course, there is also the Treasure Goblins. They try and run from you the moment you start doing damage to them.
I have not seen any evidence that the Treasure Goblins running off screen make other enemies come towards you, but that's a bit hard for me to test because when that happens, I'm usually close on the heels of that goblin, anyway.
Can enemies that you've aggroed run into other enemies you haven't encountered yet, and cause them to aggro against you?
Best Answer
I am fairly sure that a mob that is retreating will not aggro other mobs outside their group. I've seen treasure goblins run head long into a group of elites or rares and our group stop and watch trying to decide if we want to charge after and the elites waited and did nothing until we walked too close.
I also know in act one some mages will run away from you (The goat guys). I can distinctly remember one running into another pool and none of the mobs attacked until an AoE spell hit.
I have no documentation on this besides personal experience however.