It seems that when playing against the AI once you hit a certain hp amount they stop targeting you. I assume this is to give the AI a feeling of "don't target the tank" programming, but once you hit that hp level you can literally run into a lane by yourself with all 5 AI there and they will not target you. Countless times I was able to just walk up with Pudge and just slaughter all 5 heroes without them casting any stuns or damage abilities even though I was the only hero even remotely close.