What can cause the front wheel to lock up when going over a small bump?
If you have quick-release or bolt-up front axle that is not properly tightened, then going over a bump could cause it to come out of the fork dropouts. Normally dropouts are equipped with lawyer tabs to prevent this, but they aren't 100% effective or your bike may not even have them. Once it drops out of the dropout, the fork would then no longer be supported and would drop onto the tyre brining it to an abrupt stop, likely catapulting the rider over the bars.