This was "removed" in a previous snapshot, skeletons fighting back, but if the skeletons is wither type and it's in a closed space it will attack the dog once, it may have other circumstances it happens on/with and may be with normal skeletons too.

I was trying to recreate red-riding-hood story in Minecraft and I noticed if a tamed wolf attacks a skeleton it drops rare drops and XP so I made a wither farm with setblock ~ ~ ~ mob_spawner 0 replace {EntityId:Skeleton,SpawnData:{SkeletonType:1,Equimpent:[

]}} and it spawned some wither skeletons I trapped the skeletons in a box the dog/I can get out of but the skeletons can't and the XP comes to me but the Skeleton killed my dog. I repeated this and about 95% of the time this doesn't happen.

I'm not sure this only happens with wither skeletons.