This bug started in the snapshot 15w51a and has not been patched since, I have done numerous tests and cannot isolate the problem, however it seems to be due to large numbers of zombie pigman trying to path towards a player (unreachable) and one falls and causes huge server or game lag making the game unplayable. However trying to recreate the problem is very hard and the only instance I have seen it is from Snocrash's gold farm above the nether. (https://youtu.be/NK1QQ9AoTQ0) There is also redstone nearby, however I do not believe this affects the lag...