Here's the article: http://www.bikemagic.com/news/articl...N=3322&SP=&v=1
In a nutshell: Due to the standard rear-mounting of disc calibers, there is reaction force trying to pull the axle out of the dropouts, when the brakes are applied. This can overcome the retention force of QR, or cause the QR to loosen during a ride. The faster/heavier you are, the greater the force. Lawyer tabs help, but the QR can loosen to the point where it will pop over the tabs.
Looks like Hope & Sachs had it right with their early designs, which had the caliper in front. This caused the reaction force to push the axle into the dropout.
Legal issues may cause bike makers to be very slow to change this, since a redesign implies that the old design was somehow flawed.