Maybe they're just annoyed with their "commander and Chief"?
That's an incredibly stupid theory for sure.
They were acting the same way when Bush was in office.
The problem is that most of them are ignorant, really young, trained to be violent killing machines, and they have a sense of entitlement because they think the US military is so great. And nobody teaches them how to act decent, they just think they are rock stars and act like pricks. The military is obviously not training them on have to act respectful and represent the uniform with dignity.
And when they get into big groups they act like a herd, and all of them act fucking annoying with a "we're gonna take over this town, get wasted and fuck all the women" kind of attitude. It's a disgrace. They are always walking in packs, talking shit to people, blocking the roads so cars can't drive through. They are a menace to society the the areas nearby US military bases are always cesspools of crime.