The force of a half dozen people, violently thrown and/or falling against both barriers would seem the only likely way to bend it like that. If it was a pressure wave alone, I would think that the chairs behind as well as the mailbox would have been more disrupted, all having much greater surface area. I just don't think it likely that the force could have come from in front of that barrier.
Those poor people.
I’d definitely think the blast was behind the barrier, else it won’t bend the barrier toward the street. Possibly, an isle of sorts naturally formed behind those at the barrier with people densely packed at the standup tables/chairs, so dense as to leave the chairs standing after the people were treated and/or fled...speculation.