I have a stack of 21 boomalopes, muffalo, and elephants all attempting to eat grass. The biome is arid shrubland so although there is plenty of grass, there are gaps between it.
I would guess it's something to do with not being very random when selecting the next grass target, but that would only sustain the animal stack, not create it in the first place. ???
Screenshot: (http://i.imgur.com/ngbymFS.png)
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Known issue, thanks for reporting anyway.