The reason behind it is actually on old BMX thing. The tire on the front is wider and has a larger contact patch which allows for more grip on the wheel that steers and more slip on the non-steering wheel. Plus, less rotational weight on the rear allows for faster 'wind-up' and acceleration. Another way of doing it, minus the reduced rotational weight, is to run LESS air in the front tire and MORE in the rear tire.

Mike