In short the mob doesn't like a legal vote that passed (albeit under different quorum rules) and decided to occupy the building.
And, from the comments: “They intend to physically keep the legislature from assembling tomorrow to vote. Mob rule… ”
Also: “I guess that this means the new civility bullshit is officially over?”
And, noting the DNC’s involvement in these protests: “Barack Obama paid for, organized, and is putting on this riot.”
There's more here