What is happening to this wooden floor?
Looks like the edges of the annual growth rings are separating. You can read about it here Raised Grain — It's Causes
and Prevention. Unfortunately there is not much you can do after the fact, other than wiping affected areas with a damp sponge to raise the grain further, let it dry and then sand it back to stable wood.
Here is another extreme example from my own deck. You can see where the front part had lifted and broken away some time ago, what's left is only held in place because it too thick to break off.