I don't know why it does it. I had Optifine and CustomOreGen crashing because they both used multithreading and I had to disable that in Optifine. As soon as you put them together with Better Foliage you get this crash.

crash-2016-11-04_09.45.53-client.txt

I might try and isolate the problem later tonight if I have the time.

Updating to the latest version of Optifine (HD U D2) and disabling Smooth World (multithreading) in Optifine has fixed the issue.