I was at my wither skeleton farm (not recommended to be used for experience ) to get skulls for beacons (and to see how efficient experience-wise it is, which it is pretty bad, because all the EXP you get mainly goes to repair the armor afterward, but I did go from 102 to 104 levels, [random info: the 100 levels were from my skeleton farm and guardian farm from before I upgraded to the 15w50a snapshot]), and a normal skeleton shot a pigmen. All the pigmen swarmed him, and then swarmed me (I know this is another bug, MC-69032).

I went in creative to try to reproduce it, but then they all swarmed me, so I typed in /kill @e[type=PigZombie], and not just some but all the pigmen dropped XP and rare drops, but they weren't even killed by a mob!