Link: That Time a Guy Tried to Build a Utopia for Mice and it all Went to Hell

Possible explanation: After a certain point of population growth the mice cannot keep track of all the individuals in their group (i.e. in the enclosure) and in nature this would usually result in a non-violent split of the population into two groups that go their own way. But with nowhere to go the mice created smaller subgroups that couldn't go anywhere yet still tried to maintain their own territory, mates etc. which led to conflict.

This would be analogous to humans having the mental capacity to keep up relations with something like 150 to 300 people. If primitive hunter gatherers grew beyond that limit it would result in the exodus of some part of that population into a new group that went elsewhere, probably mainly to retain group cohesion but in part also because the resources needed for survival at any one place were limited. The main reasons why we have not descended into intra-tribe violence once we started to live in permanent settlements and larger villages are probably social innovations as well as recognizing and sharing the benefits of larger scale village structures. Both these things seem to be beyond mouse brains.

Just brainstorming here.

It doesn't really explain what happened though?