Why does the water level in floor drain fluctuate when the toilet is flushed?
If your plumbing is up to modern standards you should see very little movement in the floor drain's trap water level. The fact that you do probably indicates poor or blocked venting, resulting in a vacuum situation. It's not really a problem, though, unless the flush pulls the water completely out of the trap, allowing sewer gas to escape into the room. 

It's possible that there's a partial blockage downstream, but unless this is a new development it would be my second guess at the cause.