Ratchet straps shouldn't back off. The issue is how the SxS is tied down and bouncing around on the trailer. Going down a gravel road the machine will bounce all over the place. Ideally you can secure the tires and let the SxS suspension move. The second choice is on the suspension by the tire where there isn't much movement. The third choice is on the frame but you really need to compress the suspension so the SxS can't move.
For something like a SxS a 1.5" or 2" strap should be used.
You should also google "vehicle tie down instructions" for suggestions.