Unusual Procedure at DFW

Over recent Thanksgiving weekend, I went to San Jose with one stopover,
Dallas. I noticed and unusual procedure at Dallas. After boarding the
American S-80 (MD-80), the plane was ready for a push-back. All of sudden,
I heard engines roared loud as the plane push-back at higher than normal
speed. I was confused at first, but became clear when I spotted a S-80, that
parked next to us, used the same procedure. They do not use the tug to aid
the push-back. They rely on engines to push the plane back. Same thing
occurred on returning trip. This must be a common practice at Dallas.
Anybody know why they use this procedure. I thought this procedure is banned
due to ground deaths. This information is based on early jet age period.
Toks Desalu