Why is my drywall mud cracking when dried?

My friend wanting me to use 1/4" oak instead of drywall on a wall. I taped and floated it using fibergl tape. It looked fine for a few days but then started to get a hairline crack on seams. What is the cause of this? Was it using oak plywood or something else. I dont think I put the mud on too thick. I also put 3 coats mud on as usual. Please help me!!