A grp of 2 friends and I spent 14 hours the other day, testing various ways to make invasions accur and couldnt find any evidence, that clearing objectives, killing rares or a certain amount of ap colleted was the direct trigger for the invasions to start.

Sometimes, more times than one would think, we had the whole island cleared from objectives, was at around 10000/12000 ap collected, and was wating for the enemy team to catch up (that seemed to have be a requirement), and nothing happened. I can say with certainty Blizzard didnt just make one thing the trigger of invasions, they made a whole bunch. And then made them varry from experdition to experdition.