All three class aircraft with the exception of the A380 board first class passengers through the separate bridge towards the front of the aircraft (door L1), while business class and economy class passengers board through the bridge towards the back of the aircraft (door L2). However, on two class aircraft business class passengers can board through the front bridge (door L1) while economy passengers board through the other bridge (door L2).

The reason I've herd behind this method of boarding is because EK doesn't want the floor in the front of first class damaged, as well as the associated damage of having bags drag past the suites. I guess it was adopted by the rest of the fleet. Perhaps the boarding method varies by station, but this is how its normally done in DXB.