From the names that have been mentioned it appears that a lot of the leaders of those holding the federal facility - Bundy, Finicum, others - were in this convoy. It's likely that the operation was launched with the intent of grabbing them all at once and ending the standoff by cutting off its head. Hence the confrontation.
And they were going to a meeting with a sheriff and would have disarmed during that meeting
And the so-called “federal building” they were occupying was a firefighter shack in a park closed for the winter with no one around.
totally unnecessary