Here is what Champion trailer says on their website...
http://www.championtrailers.com/techsup.html
When only one brake axle is being installed on a tandem axle trailer, the brakes should be on the rear axle for maximum system performance. Rotational torque applied to axles during braking shifts the equalizer and applies more weight to rear axle. If brakes are installed on the front axle, the wheel may skid during braking. Brakes on rear axle are more effective as the equalizers shifts and the rear tires dig in to the pavement.
To me it makes sense if you consider torque and not linear force. Like why you put ladder bars on your leaf spring hot rod. But on torsion srings I can see the weight transfer to the front axle.