@Sophie1844
Wooden chairs are prone to do that. The glue dries out, things begin to shift, and suddenly someone is sitting on the floor.
I have hand-made oak chairs, very solid, but after a cold dry winter I have to check them.
If one starts to feel wobbly it’s time to reglue and clamp some joints.
I’ve never had a leg split in half lengthwise but I can see how it could happen, especially if the wood had an internal defect.
I don’t see how sitting in it would cause that kind of damage.
Normal wear and tear would be an appropriate diagnosis, I suppose, but I would guess that there was a pre-existing defect.