Two lines of zenscript did this. I thought I removed the code that spawned these trees in but forgot that there were two other lines that also needed to be removed or it would default to spawning them in any biome where oak trees spawn. i.e any biome fulfilling certain criteria like forests. And I had disabled BOP (which supplied the leaf blocks) which resulted in tree tweaker defaulting to dirt as the leaf block. And because trees can spawn on dirt, itd stack one tree on top of another on top of another etc. until it ran out of space and time to do so during terrain gen. The result was this fugly tree stack forest a good 20 trees tall
Looks like termite hills.