Garry, that looks delicious! I'd guess they would have fallen apart if you cooked the a little longer. They are hefty chunks of meat and would take some time to reach the right internal temp to fall apart.
If you have an instant read thermometer, cook until you reach an internal temperature of over 200 F in the meat. I usually shoot ofr 205 - 210 F. Guaranteed to fall apart.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan