They don't have to fall off or out of the trailer. Back in the late 70's I was following my brothers home from our local track, when the trailer hitch let go as they went through a fairly deep dip/spoon drain. I watched ( from the safety of my IT250D which I was following them on) in amazment as the safety chains snapped, the draw bar dug into the road flipping the trailer end for end, and the brothers good old HT Holden got thrown completly off of the road into the scrub. The trailer ended up upside down in the middle of the road. When we flipped the trailer back onto its wheels, all three bikes, 2 x PE250B Suzukis and a 1976 XL250 Honda, were still strapped perfectly in place, which just showed how tough Ancra tiedowns were. The three bike all had their handle bars, instruments and fuel tanks flattened, but were other wise ok.
CJ