Hello developers,
I believe I found a bug in the snapshot 16w41a. It dosen't seems to be a regular change and there is no trace about any similar change in the "release notes".
Skeletons are spawning all over the nether. They are more then every other mob and they do not require Nether Fortress in order to spawn. Considering that the light on the most of the blocks in the nether ( those, which obviously are far away from lava or fire) is 0, their spawn rate become very high and the whole world is like a giant skeleton farm.