Disc mower belts

I have a 2012 Kuhn GMD 700 GII mower. I replaced the belts about 10 acres ago. I tightened the nut on the side as manuals states. After 6 acres I snugged the nut back up til cap was just pushing spring in then cut about another 4 acres. Started to cut today and snugged bolt up a little more. Cut about 2-300 yards in a line and it started making a weird noise coming from belts. Pulled cover and 3 of the belts are side ways. What causes this? I thought I was keeping enough tension on new belts. I see where they should only bend 3/4" in the center and they don't bend that much.

Disc mower belts