Every butt is different, but so is the smoker, etc.
It probably would've been slightly more moist with another hour on the rest (in the foil).
I've had this happen before when I was smoking too low and too slow, but you said 6 hours, so I'm thinking your cooking temps were around the 300 mark, yes? Perhaps too high? I dont know.
Another approach would be to foil mid-cook.
Hance - Lake Dogs Cooking Team - MiM/MBN/GBA CBJ and comp cook
Lake Sinclair, GA (strategically about an hour from everywhere)