Pratchett himself stated an intention to commit suicide before his disease progressed past a certain point. "To jump before I am pushed", I believe was the phrase he used at one point.

http://www.theguardian.com/society/2010/feb/02/terry-pratchett-assisted-suicide-tribunal

The BBC claims that he didn't take his own life, and given his advocacy I think that his family would have been honest about his suicide if it were one, but it's a reason to look more closely at least.