Danger of shells popping up while in motion?

In the "reference area" of this forum, there is a pdf copy of the owners' manual which also has some lists of things to always carry on each trip. One of the items mentions is "Emergency shell hold-down strap (in case of latch breaking)".
Have there been incidents of this happening? The previous owner of my 3023 had a large ratchet strap over the rear shell which he said he had there "out of an abundance of caution". It seemed like excessive caution to me since, but now I begin to wonder. Have the shells EVER popped up while the trailer is in motion?