Bubble forms on 6" x 48" sanding belt

I've a combo 6"belt/12"disk Grizzly G-1183 bench sander. Every time I put on a new sanding belt a bubble quickly forms near the center. It wore an elongated U-shape in the metal table where it rubs prbly the 1st X I used it. Any thoughts on what's going on here? These belts have not been stored on any type of hook or whatnot that could weaken this area. I've thought about loosening the spring tension on the belt @ the end of the day, but figured I'd 1st see what others say. Thanks.