Actually the reason it starts to swerve is because the boat trailer is moving faster than the tow vehicle; at least that is the impression I get by the fact the boat tires are turning faster than the tow vehicles (swervy line longer than straight).

Personally I think had they applied a little trailer brake (not truck brake) going down the hill, it wouldn't have been able to fishtail like that. Probably shouldn't go off what a random internet stranger says, but while I've had a couple trailers get squirrelly in my life, none have done that.