It was against the rules of college football as recently as 2004 or 2005, when USCw played Notre Dame and Reggie Bush famously "helped" Matt Leinart forward into the end zone on goal line play. It was still illegal then.
A couple of years later, it was no longer against the rule.
So my memory isn't perfect, but I'll guess the NCAA changed that rule somewhere around 2007 or 2008.
Surprised we haven't seen more rugby-like mauls along the lines of what our offense did the other night.
Go Vols!
EDIT: Just googled it. The old rule that prohibited pushing was: Rule 9, Section 3, Article 2b of the NCAA rule book, which used to state that "[t]he runner shall not grasp a teammate; and no other player of his team shall grasp, push, lift or charge into him to assist him in forward progress." Today, that rule states: "The ball carrier shall not grasp a teammate; and no other player of his team shall grasp, pull, or lift him to assist him in forward progress." Pushing and charging into the ball carrier are now ok. Grasping, pulling, and lifting him are still violations.