Simcha, there was confusion; there were possibly events in previous 30 minutes that predisposed ICE to be on alert in general or for this particular person.
Bottom line – if a group of people start interfering and stopping armed ICE and while having a gun on you, 1 in 10 or 100 times something might happen.
Yes, they were trigger happy, but in the situation that called for it. They are confronting an organized group, they have to expect the worst at every moment and act accordingly.
Note that in almost all cases when there are incidents as such, the person who gets shot usually has some history. Statistically, if ICE or police were out of control, much more of cases were about totally innocent – bystanders, peaceful demonstrators.