Honestly this all could have been solved last night.
What they should have done easily, when Negan was at the gate have like 10 people with rifles ready to fire asap. He couldn't see if anyone was behind the gate. Open gate, and then BOOM. All his guys wouldn't have expected it.
It does feel like a booby trap or something could have been set. I'm guessing the fact that they still held Daryl was preventing Rick and Co from doing anything drastic.