Sources close to the hubby of the 'RHOP' star tell us Gordon was handed court docs on Mia's behalf back in mid-August ... which signaled her desire for legal separation. We're told the move completely blindsided him -- and that he had no clue it was coming.

Our sources tell us that Gordon is also peeved right now over Mia's public handling of the situation -- namely because we're told he feels like she's been BSing everyone.

Fact is ... until Friday -- when MT announced they were separating -- she'd made it seem like they were A-OK on social media, continuing to post photos of the two of them together and even shouting him out a handful of times ... calling him a positive influence on their kids.

Our sources say Gordon is irritated by this ... and not just that, but we're told he hasn't even signed the papers yet. Her going public can be viewed as pressure on him to do so.

Mia broke the news to People, saying ... "Like all marriages, ours has had its ups and downs. At this time, Gordon and I are separated, but my family is my number one priority, and I am committed to doing whatever it takes to make sure we are okay. We ask that you respect our privacy and give us the time and space to figure things out."

Of course, the backdrop of all this is the fact that "Real Housewives of Potomac" has a new season on the horizon ... it's set to premiere in November, and this is sure to be covered.