Usually, the type of person who does this had a less than glorious job in the Military. Many haven't even served in the Military at all.
Real SEALs are humble about their jobs, and do go around bragging about being one. Phony SEALs are the opposite. They can't wait to tell someone that they're a Navy SEAL, and will go on with endless bullshit stories about it. They like the "ooo and aaahh" reactions that people give them. They want to intimidate people and get respect. All because they're not winners, and never did anything notable in real life to earn any respect in the first place.
"What class were you?"
"Your BUD/S Class....?"
"20 Bravo Delta 1"
"You fucking idiot, you're a Phony Navy SEAL! Now I'm going to smash your face in!!"