I am curious on what sort of tree this is too, tree hollows can be natural or artificial, and based on the shape and smoothness of them these are natural. Natural hollows are caused by damage to the tree (branches falling off, insects, or elemental damages such as lightning) and these look like branches but for a cluster like this is at the very least rare and probably means the tree is incredibly old unless there is some special tree species that has properties that make them faster.
The better question is what the hell is up with this weird-ass tree?