check the front wheel alignment. you can't always just eyeball it, so what i usually do is stick my index fingers between the rim/tire and the fork blades to feel it.

there was a girl who came into my shop a few weeks ago with this beautiful kestrel talon with 9 speed DA complaining of this. shop manager said to check her headset, so i did and it was brindled so the fork would resist moving anywhere but straight. i checked her wheel alignment and sure enough, it was out. then i told her about the headset and she was all "OH NOES!! WHAT WOULD CAUSE TAHT!?!?!" and we explained that impacts, or crashes would do it and she said she's never crashed it. after a few moments of what appeared to be deep thought she asked "hey! i've been practicing wheelies! would that do it!?" i just shook my head and walked away to find her a new headset.