1. Guy One
I married this prick! Like you, just objectively, he WAS punching above his weight. So it wasn't embarassment over ME.
I saw the same attitude in his father (who was also married to a fairly pretty woman)
When out in public - he'd just get overwhelmed with resentment and then have a massive mood swing and get his whole family to show solidarity by "being as unhappy and uncomfortable as him.
Whether it was everyone leaving early, or just stopping talking and sulking/stonewalling. I'm actually a fairly charming/sparky type whose natural mode is diplomacy/smoothing over, but this man was a total life-drainer.
It came down to a sense of resentment at not being the man he thought he "deserved" to be in public? Everything reminding him of his own failings, so the internal rage he felt was released.
2. Guy Two
I dated this guy. Again, that nasty inherited air of "I'm uncomfortable, so I need to make someone else uncomfortable". It was me, or the waitress, or the barmaid, or that guy who'd brushed past him. Toxic. I dumped him after a month, learned my lesson ha ha!
I can see you're thinking "poor lamb, he feels awkward."
But you know what?
I've dated men who admit they might not be comfortable in a social location (I'm vair cultured so often I'm suggesting going somewhere a lot nicer than what they're used to).
But they just put on a brave face and shake hands and act politely like my date and then after say "gosh, I felt well nervous there ha ha!"
Personally I've often been at work/social functions where "I'm that odd one in the corner no-one wants to talk to". Or where everyone is cooler and younger and prettier than me. Or no-one pays me attention 
I DON'T find someone to shout at or bully, I sometimes drink a bit too much, or I find something to do, or just leave and say to the host "lovely event" and go to myself "wow, that was hard."
Politeness is the default mode of normal men/people.
So I think your partner is an angry raging type who DECIDES to be a twat, sorry.
Do a search for "two modes nice or aggressive" or "waitress test" in the search box, there's some good threads there. Also "predatory self-esteem"