I hired a flooring contractor to refinish my floors. He just finished and there are noticeable grooves in 1 room. These grooves are about 1/2 wide and run the length of the room. You can see them clearly in this photo:

https://photos.app.goo.gl/ssAnqviXeeiDbMmL9

I am pretty sure the grooves were not present before the floors were refinished. Even if they were, I would have expected the sander to get rid of them.

My questions:
1. What could have caused this?
2. What can be done to fix it?

I have never seen this type of mark on a newly refinished floor. To get the floor flat it will need to be sanded again.
I have no idea how these grooves were made. It must have been during the sanding process.