Great question, I think this is one of the reasons I peaked in velocity for a while. I'm not a very tall pitcher so in high school I felt I had to be the strongest to make up for my shortcomings (no pun intended). So I was always focusing on getting bigger and I lifted with the football team during the summers and I didn't focus on pitching in itself as much. I took my mechanics for granted. While my bench, squat, and power cleans all went up tremendously my velocity was still the same. My first real jump in velocity came was when I spent the summer working out the kinks in my mechanics. I saw a 2-3 mph difference in just 2-3 months.
On the flip-side of things, there was a kid who came into my college last year throwing 86-87 in the fall. He never touched a weight in his life, he was about 6'2" 170. At the end of the fall after he had worked hard in the weight room, he gained about 10 lbs and his fastball went all the way up to 92-94 consistently. Not saying that will happen to everyone who prolongs weightlifting but just an example of there being two-sides to everything.