Putting the tongue weight of a pontoon trailer on an ATV will certainly leave the front end light and make the rears dig in. Most of the Polaris atv's have a towing capacity of about 1200lb. I don't know the rated tongue weight but I'd bet that it's 300 or less. You probably had the fronts spinning along with the rears.
Less, probaby 35-50pound tongue rating. What year is the Sportsman 400?
