AI Threat.

So the other day I was helping someone out with their map.
In that map, Computer controlled enemies always went after players first, however when the armour type of allied units was changed to "Heavy", the enemy computer started attacking non heroes first.

Ok that was phrased badly. What Im saying is that Heavy armoured units tend to get focused first.

So the question is, what actually does define the priority in which an AI attacks a unit?
That of course is excluding obvious things such as Taunt ability etc.