As a guess, I would say that either Nancy thinks they are currently vulnerable and need protection OR they are about to do something that will require protection from p!ssed off Americans.
That fence wouldn’t stop pissed off Americans. It, like mask-o-rama, is political theater.
My predictions:
Gun control and confiscation EO
Ammo registration and tax EO
Declaration of national emergency powers to force the Green New Deal and nationwide lockdowns to “save the planet” enforced by the military ignoring Posse Comitatus Act
Amnesty EO