Why are airplane passengers boarded from front to back?
A number of airlines (including Delta) use a 'zone boarding' procedure. In short, the first/business class board first, followed by 'blocks' of passengers, usually from rear to front (In your aircraft, this was apparently done the opposite way).
However, it is important to point out that the back-to-front is not very efficient, atleast if you take into account the time taken to seat everyone. Experimental evidence indicates that the back to front method is one of the least efficient of all the boarding methods. The table below shows the time taken for 72 people to board a (mock) airplane using various methods.

Table from Experimental test of airplane boarding methods by Jason H. Steffen and Jon Hotchkiss
The basic problem in following back-to-front method, according to the author is that it is a serial process:

The passengers would rush into the cabin, proceed toward the back—and come to a screeching halt as the first one or two passengers stowed their luggage. The first 30 passengers (the back five rows) would take up nearly the entire length of the cabin. The rest of the line has simply moved from the airport gate into the jetway or cabin—and it moves no faster.
... In this case, only one passenger at a time is seated. The aisle in the airplane isn’t used effectively.

There has also been studies on optimal boarding procedures, which indicate that optimal procedure is different from what any airline follows.

There was a mythbusters episode that simulated boarding an airplane using various methods in vogue. Making allowances for the lack of scientific rigor, the results of boarding 173 passengers were pretty much the same- back-to-front is the slowest method.

Image from mythbustersresults.com