The protestors fighting with the police started on the EAST side of the building, immediately after Trump's speech and before the rally attendees could march to the Capitol. The Trump Rally marchers were approaching from the WEST side of the building. They couldn't see what was happening on the East side. The people who started the violence with the police were on the EAST side of the building and DIDN'T ATTEND THE SPEECH. They weren't there for Trump. Who goes to Washington DC and doesn't even care to see the President speak live? Take a look at the Capitol building on Satellite view. If you're walking from the White House to the Capitol, you will approach it from the West. By attacking on the East side first, the infiltrators could mask their violence from the true Trump attendees approaching from the West. They didn't know what was going on until the Police at the capitol suddenly started pelting them with tear gas as they reached the steps. Everything was timed to bring the Trump supporters into a trap that had already been set.