United Passenger Was Resisting Standard Procedure for Overbooked Flight According to Airline

United Airlines staff was merely following "established procedures" when security was called to remove the passenger ... according to United, which is finally explaining its side of what went down before the video.

TMZ has learned United CEO, Oscar Munoz, fired off an email to all employees Monday afternoon after the airline was blasted on social media for the way the man was dragged off the jet. Munoz explained the crew "sought volunteers and then followed our involuntary denial of boarding process (including offering up to $1,000 in compensation)."

The flight was overbooked, and 4 passengers needed to be "re-accommodated" -- to use United's word.

Munoz added in his email, "When we approached one of these passengers to explain apologetically that he was being denied boarding, he raised his voice and refused to comply with crew member instructions."

Munoz says the man got more and more "disruptive and belligerent," and continued resisting ... running back onto the plane.

The CEO makes it clear he supports the actions of his United crew, but says "there are lessons we can learn." The Chicago Dept. of Aviation has already suspended the officer who forcibly lifted the passenger from his seat, leaving his face bloodied.