I don't read the comics so I don't know how close they are to the source material for the Alexandria storyline but I thought the way they set up season 6 was really cool. I wasn't thinking about the story arc during my viewing so I didn't think it was going to end up THAT way. Pretty cool. My guess is
that nobody intentionally set off the alarm. Somebody accidentally sets it off stumbling across something. Doesn't seem to make sense why anybody from the community would want to screw up the whole plan. If anything, aside from the crew running it, everybody else was under the impression it was going to be a dry run. My guess for the next major character death is Glenn. They're setting him up as another guy "too good to survive in the apocalypse."