Could be the underlying plywood (or whatever is under it in that area) has separated/gapped at the seam so the hardwood goes along for the ride too. They too shrink with the dryness of many homes in the winter months.

The again maybe the last few rows were glued and the glue bond gave way.

Darned tight fit against the slider too. Hate to see it in the humid summer months unless.... When was it installed? Recently?

That's my guess from probably a thousand miles away.

Ken Fisher