I discovered something this morning. I put another bow on the tree and the strip of wood with the DL written on it was laying on the floor. My shooting machine sits on a workbench kind of under the tree. Room constraints mean that if the back of the bow were straight it would be about 4" from the measuring strip. Once it's braced there is 6-8" clearance. The string had to have come off in order for the bow to bend backward enough to knock the strip off my tree. The picture shows how much clearance there is with a similar bow. The bow is not braced. There is 5.5" to the strip. It had to bend forward a long way to knock the strip off. It probably broke before it hit the strip.
PS The strip wasn't very well attached