The boats on trailers with masts DOWN were fine. They all had sail boxes and were pretty heavy. A Capricorn with MAST UP, on a heavy trailer was fine.
A H-16, with mast up, on the trailer was fine.
My boat, mast UP was tied to the trailer and flipped anyway, trailer and all. My rigging hung-up on the trailer of the Capricorn parked next to me. My boat suffered no damage, neither did the Cap. My trailer does not have a sailbox. I have intentionally tried to keep my trailering weight down.
I will add a sail box to my trailer, with very little consideration of weight and keep some tie down anchors in it. My preference will be to keep the boat on the trailer with mast down.
Was it tied down on both sides ?
I always try and tie my boat down both sides if I can.
So <boat trailer><BOAT><car or other trailer> and then use the trap wires with ropes on to tie the boat down; this stops it blowing over. If at all possible I will also try and tie it to something to stop it going forwards or backwards.