I agree with BGFisher that it may be a spine problem. Also, a diagonal pin alignment also indicates that your centershot is off from what it should be. Set up a paper tuner and let a few rip through that at 4 or 5 feet. If your tear isn't close to a "bullet hole", this will be where you want to start.

Before I had an understanding of paper tuning and centershot I shot pins that were on a diagonal. This was, of course, with a sight that allowed individual adjustment. On a well tuned bow, the pins should be a vertical straight line.