I've seen the exact same thing and it was just as Steve described. Roller trailer!

I remember my first roller trailer. Got ready at the launch, removed the tie down strap, went to back in to the water at Ludington and felt a jerk. Thank god the I was smart enough to leave the safety chain on! I learned that you cannot unhook that joker until after you are in the water. Which would later be a pain in the butt and force me to completely rethink my life choices! They have their advantages with bigger boats like the 24 footer I was launching but I much prefer to launch the boat from the comfort of my truck seat... Come to think of it, I don't launch from my truck anyway. I sit in the boat and have someone else back it in the water!

My guess is they put a couple slinges under the boat and got a backhoe with front end bucket or a front end loader to put it back on the trailer.... OR, the guy simply hooked a tow strap to it and drug it to the lake. This is the UP afterall!