It all went down Sunday at Summerfest in Milwaukee -- sources close to the band tell TMZ, Eric Wilson's rott was pulling him around backstage on his longboard ... when the bassist took a nasty spill.

We're told Wilson's hand had somehow gotten tangled in his dog's leash -- and the strong animal dragged him 10 feet, dislocating his right shoulder.

Paramedics responded to the scene and rushed Wilson to a nearby hospital, where doctors popped the shoulder back into its socket. Eric's now back on the road, but still recovering.

Lucky for Eric, he didn't injure his left shoulder, which supports his bass strap and does most of the moving when he plays. Eric's rep tells us, "If he had injured his left shoulder, we would have had to cancel the rest of the tour."