I've read posts in the past from people who were told pontoon boats are too heavy to bounce off a trailer. Well not so fast. Those transom straps may have prevented this.
I'd venture to say that the pontoon wasn't on a bunk trailer, but rather the narrow, tippy, and absolutely worthless for anything but marina use type of trailers. I cringe every time I see one of those trailers being hauled down the highway. It's an accident just waiting to happen. There needs to be a law that limits that type of trailer to 25-35mph max and to be used on rural roads only.