Crash 100% CPU OOM Exception on Ticked Skeleton

Just before this happened there was a skeleton in the river outside my fort and several blocks outside the fence. Whenever I got near the fence the skeleton would shoot arrows at an unusually high rate – the knock back was making it difficult to approach, so I went back inside to get a bow. When I returned I heard skeleton sounds but couldn't see it anywhere. Eventually the sounds went away, but soon the CPU went to 100% and the memory heap started growing rapidly.