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
Not just that but technically I can make the leaves, log and the block the trees spawn on anything I want so I could use this to make ice/obsidian/stone tree towers or whatever too.
One issue with this is that tree tweaker doesnt care what your structure settings are. It will spawn trees on superflat maps just the same as it would a normal map.
Thats actually how I accidentally did this. I was porting the tree spawning to biometweaker and didn't fully remove the zenscript that spawned them in.