Looking at FMLCommonSetupEvent, the compostables map is updated. The map itself is a hashmap being updated during an asynchronous event. As such, a CME can be thrown if another mod, say for example, Twigs updates the compostables map at the same time.

This can be fixed by wrapping the non-thread-safe code using #enqueueWork provided by the event.

This issue was reported in the following Forge forum post.

Fixed, thanks for the help!