I am suprised that you thought my thumb was coming out soon....
It looks to me like it's falling off the thumb. Maybe you need a better grip, meaning one or more of these:
1 - tighter thumbhole
well,, upon watching a second time, I see that your problem is that your hand is on top of the ball. Sorry. That made me think you had the hand under the ball and then the thumb slipped out. However, now I'm thinking that you never even had the hand under or behind the ball to begin with at any point in your swing.
You have to get your hand behind and under the ball. I would follow the suggestions Atochabsh made, and buy a 10 pound Kettlebell at Target and build up your wrist strength if you don't want to bowl with a relaxed/limp wrist. You should be able to hold the ball with your wrist in a firm position - fingers below the equator - for 10 seconds.
My wife bowls with a relaxed wrist, and she gets good axis rotations. I can't tell you how she does it, though.
I have found a way to bowl without muscle, and will post a video maybe in January, when I get my replacement flip. It only took me - let's see - 50 years.
"If it ain't workin', you're either throwing the ball wrong or throwing the wrong ball."
"Follow the oil!"
"Dry lanes ain't worth a shot!"
"I love the smell of lane conditioner in the morning!"
current avatar is Gabby Hayes. Looks a lot like me!