Camber. If you have too much Positive Camber it will cause the tires to wear on the outside edge. This could be due to a lot of different reasons including boat weight, bearing issues, bent or damaged axles. I would get it in to a trailer shop sooner rather than later.
Issues could also be from a Caster or toe issue if your axle is off center of your trailer. If that were the case you may expect to see wear on the inside edge of one tire and outside of the other, with evident un-even wear across the center treads as well.