Marquee Tool Glitch

Lets say for example, you have a layer for a background, and then a layer on top of that background for trees. If you marquee and copy paste a tree and place one(while selected on the tree layer) and move over to a different frame of your animation and return to the frame you originally placed the tree( IE: from frame 2, to 1, back to 2), it will now contain parts of the background layer in your marquee selection. I’m not sure how to make a video to show this, does anyone know what I am talking about?