May be leaning to compensate for a fork or frame that's out of alignment? When the bike is vertical, not leaning, it's pulling to the left? Maybe have an experienced cyclist or even a frame mechanic check it out to confirm. If it's steel, can twist the frame or tweak the fork to fix. Tweaking the fork alone can compensate for a twisted frame to some degree. Lots of "how-to's" online, and depending on where you are there are probably a few qualified people around.