NY Jets Geno Smith -- 'Sucker Punched' Over Charity

The Geno Smith locker room brawl went down because the Jets QB was AWOL from a charity event, which pissed off his teammate ... TMZ Sports has learned.

Sources inside the Jets locker room tell us backup linebacker IK Enemkpali -- the puncher -- organized a youth football camp and charity in Texas last month. He invited Geno to attend -- even put his face on the flyer -- but for some reason Geno didn't make it.

IK was also pissed because he paid $600 for Geno's plane ticket to TX.

We're told IK stepped to Geno to talk about it on Tuesday, but Geno told him to back off because he wasn't going to discuss it at that moment. Geno turned away, but IK stood his ground ... and socked Geno in the jaw.

One player told us, "He straight sucker punched him." As we reported, Jets brass immediately cut IK from the team ... and Geno's out for 6-10 weeks with a broken jaw.

IK says, "Geno and I let our frustration get the best of us, but I should have just walked away from the situation. I deeply regret and apologize for my actions. It was never my intention to harm anyone."

Tell that to Geno's jaw. We've got a call in to his reps ... so far, no word back.