To me it seems like they had no search warrant and they were just going to "knock and talk"/bully their way into the place. Which is a very common tactic for narcotics agents.
My take on it is these guys got too used to dealing with people that give in cower in a corner and let them have their way. So they got a wake up call.
Why was back up so far away? Why such a small team? Was this possibly a small rouge team that likes to collect unaccounted for money and drugs for personal gain?