Two main reasons for lower back problems. One is tight hamstrings, like topvelocity said. Try being 35 and sitting at a desk most of the day, not good for the hammies and the back.
The second is in the kinetic chain, after hip rotation, after shoulder rotation, at ball launch, the back is arched and "unloads". You can see in the two pics of Maddux, in the first his back is arched, in the second he has unloaded it. Proper warmup and strengthening is essential but the other factor is that sidearm or submarine style pitchers do not translate this unloading very well and many of them have back problems. (Randy Johnson, Chad Bradford, Dan Quisenberry, Kent Tekulve). Do you have a lower arm slot?