He appears to have been removed after becoming intoxicated and belligerent at the airport, with reports describing him as being arrested for public drunkenness and caught on bodycam threatening staff and police after being taken off the plane.

What the reports say

The most consistent account is that Poirier was escorted off a flight at Hartsfield-Jackson Atlanta International Airport, then got into a heated confrontation on the ground that escalated into an arrest.

Why he was kicked off

Publicly available reports do not say he was removed for a single clear aviation-rule violation like seat assignment or paperwork; instead, they point to disruptive behavior tied to intoxication, which is why the arrest was listed as “public drunk”.

In plain English

So, the short version is: he was reportedly kicked off because he was allegedly drunk and acting aggressively enough that airport personnel and police had to step in.

Context

If this is the incident people are discussing online, it has been framed as an airport meltdown rather than a technical boarding issue, which is why the story spread so fast on sports and gossip outlets.

TL;DR: the reporting says Poirier was removed for being publicly drunk and disruptive, not for some normal airline seating dispute.