Hot Tires?

Ok now I think this is odd. I was taking my boat up and over the mountain to get some maintenance done. I don't trailer this boat but 0.5 mi to the launch ramp twice a year and the mechanic. So I am 12 mi into a 35 mile trip and I pull over to check the bearings on the single axle boat trailer. They feel cool to touch, however the tires are hot. There are no trailer brakes and both sides felt the same. Why would tires only, not the rims or hubs be hot? I slowed way down the rest of the way. Tires are old won't lie, and for sure I am going to change them before they go on this trip again.