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.

Handyman is on the right track and it is a good answer.

The reason is there has been a change of the humidity and basically the top of the flooring is separating from the rest.

Why though?

• this is cabin grade plank flooring. Cabin grade flooring has splitting issues unless treated well. Cabin grade isn't bad, it is just different.
• The floor was stained and maybe sealed (lightly) at one point. The seal has worn. It was never sanded down (cabin grade isn't on purpose). Now the section that took the stain has dried out faster than the bottom and it peels.

What do you do?

• Sand the floor down and stain it. This is really about how many splinters do you want and what do you want the floor to look like. I have see cabin grade flooring look almost like the premium cousin after its first sanding, stain, poly - heavy. If you don't want to touch it too much just sand areas with issues and restain, knowing that this will be a continuous battle.