Thanks Rico,
It was a sports medicine doctor that described how the tendinitis occurred with me. Mine was in the forearm, just below the elbow.
According to him, the tendons become inflamed due to a "new" or a "repetitive" movement. Bowling is definitely a repetitive thing, but it's especially hard on those tendons when our thumb is "locked", so to speak, in the thumb hole while we twist or rotate around the ball.
I can only go by my own experience, but when I started getting the thumb out earlier (or rotating later) the problem was solved. It's been over five years (and I'm five years older) so to me it's no coincidence.
We can only guess, from this forum, as to why you're having the pain without watching you bowl. Stretching and warming up muscles has become increasingly important as I have gotten older.
How many guys do you see throw their first few practice balls with full effort? I know my first several are at 60-70% until these old muscles, tendons, ligaments (everything else) are ready for action.