That was going to be my guess. Also, a loose front skewer can result in your front wheel falling off and you smashing your face gainst the pavement. You should check it periodically (like, say, every time you put air in your tires). They generally don't loosen on their own but sometimes tricksters can get at your bike when it is locked up, or it wasn't tightened properly during the last maintenance. Disk brakes have also been found to cause QR loosening over time.