Things always run, that's just CoH. Taunt auras just make it very much less frequent.
I know I was a bit harsh on why things run earlier, but there are a few other reasons things can run besides what I mentioned. For example, don't know if this issues end changes changed anything, but things that couldn't make an attack when they tried would run. Another thing is that if something misses so many attacks in a row, they will try to run, but usually a taunt aura will keep them around. The latter is usually what makes things run when you get them out of range of the taunt aura and they're less than 16 guys.
That all being said, things run in CoH. As soon as you make one attack, if you haven't hit them with a taunt, there's a chance they run. That's how it's always been and it kinda sucks. For example, if you're on x8 with a blaster and just throw a single fireball, a decent chunk of the pack will run.
You'd need someone with the code to explain it.