Can someone explain this cartoon to me, please--that is, the why of the second word balloon?

Others have suggested that there is something in the phrase "We, the people" which is grammatically incorrect, Jackie, but that is not what the cartoonist has in mind. His touch is far lighter than that.

What brings the guard out is the rebellious expression "We, the people". They have been dispatched to suppress the expression of freedom, not the expression itself.

It is the play between the shallowness of the cave-dwellers and the principle of free expression which creates the humor here.