This is what's generally called "glass surfing", and they do it for a variety of reasons. If you can give more information such as lighting, temperatures inside the cage, diet, etc., that might help us determine why. Be as specific as possible when going over the lighting, like what brand of bulbs, how far are they away from the basking area, when was the last time they were changed out, etc. Beardie age, gender, and any behavior changes that may have happened recently are good too.
Also, pictures will help if you can upload them using the User Image Uploader link at the top of the forum and then embedding them in the post using the XIMG button.