My guess would be the strap popped loose while driving down the road, the tire ran over the strap and tore it off. how close am I??
As far as a Upholstery place to make tarps, I can attest to Custom Canvas out of Bismarck. Monte made one for me and then another for a buddy of mine after he seen how nice my tarp turned out. He does great work, the tarps fit like a glove and are fully customized to each boat. Great guy, his dogs can be a little intimidating. If I remember right he has a couple rottweilers. I remember pulling up to his place, got out of my truck and around from the back of the shop these two rots come trotting towards me, one holding a deflated basketball in his mouth. I don't think either were aggressive but they scared the shit out me, lol. Since I got to his house a little early and he was on his way home to meet me, I decided I'd wait in my pick-up until he got home, lol.