There is widespread variability, but certain US airports seem to spend a lot more of their resources on gate screening. Denver is one of them, as is Cleveland.
Since about 2003-04, I don't recall ever seeing it in other airports that were my hometown airports: San Francisco, Seattle, or any of the three New York airports. I'm sure it's happened, but with far less frequency than Denver.
In fact, I think I've seen at least one TSA cart at a gate every time I've gone through Denver.
It is unfortunate that they picked a flight with a political element to it. I doubt we'll ever know, but I don't think that the TSA is all that clever. They might specifically have gate searches for flights to Washington/National, so that gives the unfortunate impression that they were targeting those going to a protest march.
On the other hand, you never know. Maybe the TSA was trying to rattle some protestors, but I an ID check or bag search wasn't going to deter anyone. Just look at the footage of the Atlanta police force giving high five's to all of the marchers. There you can see law enforcement clearly offering their support and well wishes.