If the brake pedal is going hard when the problem occurs, it is likely that the ABS system is actuating when that occurs. Antilock brakes work by momentarily shutting off hydraulic pressure to one or more wheels to prevent them from locking up; the feel to the driver is a momentarily hard brake pedal along with a slight reduction in braking ability since some of the brakes are being shut off at that time.

The only other possible cause for the symptoms you describe would be a vacuum brake booster failing, but usually a failing brake booster will make a hissing type noise audible inside the vehicle.

If your ABs system is actuating at inappropriate times, there could be various causes, including a circumference difference between tires, a locking brake due to a sticking wheel cylinder or caliper, a sticking parking brake mechanism, or a defective tire.

This is not an easy type of failure to diagnose at home without test equipment. A technician will want to connect a Ford NGS scan tool or equivalent that has the ability to communicate with the ABS controller in the vehicle to retrieve fault records and display live ABS system data to begin diagnosis of the problem. This is still a relatively new vehicle, it may be under factory warranty yet, so i would suggest taking it to your favorite local Ford dealer.

I hope this is helpful to you; If I can be of any further assistance please let me know!