This could be due to brake fade - the brakes may have overheated, and the brake fluid boiled - once the brakes have cooled, everything is back to normal.
When was the brake fluid last changed? And was the correct fluid used (DOT certification)? It's due to be replaced every 2 years. If the brake fluid is contaminated, or absorbed water, the boiling point will go down and the fluid then starts turning from liquid to gas which will result in significant loss of braking efficiency, until the brakes cool down again.
This can also happen on a well-maintained car, e.g. when driving fast and braking hard repeatedly ('traffic lights drag racing'), or when going downhill with the foot on the brake pedal (instead of putting the car in lower gear, or using the shift limited if automatic), especially with cars fitted with standard brakes (as opposed to the larger discs on the Sport variant).
