Diz, I think you are correct except for coming around the ball. If he did that, his track would drop and be tracking no where near the thumb. Its in his release.
A good practice is to take a video of yourself when you are bowling good so you can compare it to one when you are bowling bad.
Depending on where you live, your thumb might have changed due to the cold weather and you might have dry skin. All these things could contribute to your release problems. ie grabbing the ball, dropping the ball etc.