11:35 AM -- Shaq Lawson is apologizing for his role in the incident ... claiming he only took action after the man directed death threats at Bills players and their families.

"Those of you who know me know that I'm all about the fans, and I have a lot of respect for the game I play and my opponents," Lawson said on Monday. "Before the game started yesterday, there was a fan behind our bench, who was making life threatening remarks towards us and our families."

Lawson claims the players alerted stadium security about the situation, but the fan was not removed from his seat and continued to heckle their sidelines.

"Eventually, our emotions boiled over, and I made a mistake," Lawson added. "For that, I apologize, but there are certain lines that should not be crossed."

A confrontation between Bills players and one Eagles fan on Sunday grew so contentious ... it actually ended with Shaq Lawson shoving the Philadelphia supporter.

It all happened just before Philly beat the Bills at Lincoln Financial Field ... when a throng of Buffalo players got into a verbal argument with a man in the stands wearing an Eagles-colored beanie.

You can see in video shot by a bystander, Bills defensive lineman Jordan Phillips first got into the man's face ... while Lawson, Greg Rousseau and Ed Oliver joined in on the spat just steps behind.

The guys traded words for several moments ... when, suddenly, Lawson can be seen on the video putting his right arm on the man, jolting him backward. The parties, however, were quickly separated by stadium security after the shove.

Neither the Bills nor the NFL has publicly commented on the altercation.

Meanwhile, Buffalo fell to 6-6 with the overtime loss ... while the Eagles moved to a league-best 10-1.