This is how I found this bug:

There are a few zombies standing near each other. They aren't in a team, so collision is possible.
I changed their NoAI-tag to 1, so they are standing there all together without moving. After setting the NoAI-tag back to 0 they push each others up many blocks away from where they were standing while having no AI.

It seems as if a sort of pressure builds up when mobs without AI are standing near each other. Regaining their AI makes them fly in every direction because the tension discharges.