Spider spawner spawns a bunch of AI-less skeletons

I encountered a (regular) spiders spawner, which started spawning a bunch of skeletons after about 30 seconds. They wouldn't move or attack me, and I couldn't attack them more than once (the first hit would register, but after that I can't hit them anymore).

Seed : 361214078108
Coordinates : -1737 26 -3091
All generated in 15w41b