Dave,
The swept back wing on jets is designed for high speed flight and not so suitable for the slower approach speeds. A compromise must be reached for the approach speed, high enough to keep away from the stall, lower than the tyre limiting speed and low enough to avoid the brake energy limits. A flap failure can also be one of the effects of a larger systems problem e.g. a hydraulic system failure. The formula 1/2mv^2 should give you an idea of the increased energy taken onto the ground. 35000kg and 87.5 m/s (170knots) if you want to throw some figures together, takes a fair amount of stopping
Cyclone