The most common reason art buckles in a frame is restricted movement. Different things can cause this, but often it is that paper expands and contracts with changes in temperature and humidity. That is why it is important that you only ’hinge’ art (see mat board answers) from the top, so in effect it just hangs, and has room to move. This is also another side benefit of using a mat board rather than full framing, where the art may be too restricted.