why do both front btrake cslipers lock up at the same time?

Some possibilities I see are 1. Collapsed hoses to calipers. 2. Debris in valve or actuators. Does the brake peddle feel normal like before the problem arose? Is the fluid in the reservoir dark? One simple thing you can try is to flush out the brakes system. Follow the procedure to bleed brakes and flush some new fluid through the system to dislodge any foreign matter. It is possible that you have both flexible lines to the calipers collapsed. This in effect acts as a check valve on both calipers.