Your camera has some movable parts in the viewfinder which is basically the same as an SLR of years past.

When you take the lens off and look into the opening you'll see a mirror which is at a 45 degree angle this sends the light to the finder so you can view the image and then moves up out of the way so the digital sensor gets the light.

Most likely the mechanics of this mirror is sticking so it takes a little longer to fall back down after the picture has been taken. The bumper at the top which the mirror hits is likely breaking down so its getting gummy. You really need to bring this into an authorized Nikon camera repair center or Nikon its self. as the foam bumper pad needs to be replaced. I wouldn't try doing this my self as you need to take a good part of the mechanism apart to do this right.