I have a friend who used the minnkota battery box setup on his pontoon (with a wet battery) and it did the exact same thing a few years ago. He now has a standard box and a 50A breaker on the lug. He hasn't had an issue since and nothing else was changed. I suspect maybe the wire or connections made as part of the box were high resistance. (possibly even the 60A breaker itself?)
A 45-50 Amp draw will generate alot of heat in a small amount of time!
Chris