Any NFL rules experts here???

Watching the highlights of the end of the Bengals/Bills game and I can't quite figure out what the rule is. The Bills defensive player intercepted the pass, falls to the ground, gets up untouched by any Bengals player, and runs into and out the side of his own end zone celebrating... Everyone acts like it's a dead ball or something, but shouldn't that be a safety or a touchback or something?