Unexplained Butterflied Roast Chicken Success
It sounds like you accomplished something like braising in your oven. Low moist heat for an hour will definitely make meat fall off the bone for something as delicate as a chicken. The skin was crisped up by the broiler, of course.

You could probably accomplish something similar by crisping up the skin in a pan or under a broiler, then putting the chicken and some liquid in a crock pot (slow cooker) for an hour. Brining the chicken first helps prevent it from drying out. If you didn't brine yours, it's possible that there was enough salt in your rub to do the job.