I was wondering if anyone can explain to me a rule I was unaware of. Toward the end of today's game, St. Louis received a punt in their endzone. Rather than taking a knee, the receiver sort of shifted back and forth and was pushed out of the back of the endzone.

Why was this a touchback rather than a safety and what's the rule/rational for this?

It was a touchback because the impetus (movement of the ball) came from the opposing team (the Patriots). Hall was in the endzone because that is where he received the punt. His actions in the endzone don't matter as he is only there because that is where the Pats put the ball.

It would have been a safety if Hall's impetus had put him in the endzone - E.G. He ran out of the endzone and back in or received the ball out of the endzone and ran back into it.