mmm, this sort of thing shouldn't happen while tooling along at those speeds on the road. Commonly what causes this is the derailer shifting past the biggest cog and getting wrapped up in the spokes, which you did not describe. Or some foreign object getting caught in the drive train which is unlikely on the road, but not out of the question. Then theres chain suck, but your bike being fairly new is not likely to have developed wear on the chainrings enough to cause this and other factors like mud and such shouldn't have come up on your road ride.
All that said, we're forced to assume the thing was just not tightened in the first place and probably should be replaced under warranty. Sounds like the shop may have a hard time getting Shimano to provide a new part unless they can demonstrate that it was defective in the first place. I am not positive about Trek, but most current bikes have replaceable derailer hangers so your frame should be OK.