Ok, in this video you can clearly see:
ICE is dealing with a female protester when Pretti inserts himself between the officer and the woman.
Then Pretti initiates first contact and pushes against the officer with his arm to block the officer from the woman.
Officer maces Pretti.
Pretti turns and shields woman with his body (either that or he's just falling over from being maced or slipping on icy road)
Officers begin to pull Pretti off woman to arrest him or her, scuffle ensues.
During the scrum a gun is discovered.
Pretti is disarmed then the shots happen
I don't know why they shot. They shouldn't have
It happens so fast, but I will say, none of this happens if Pretti doesn't make first contact with an officer.
This may come as a shock to some of you (or maybe just one of you) but you can't obstruct, impede, interfere or initiate in any way an officer while performing his duties.