Here goes another theory I thought of on why the blast doors come down and whether it’s related to the food drop.
I suspect that under normal conditions there is more than one person designated to be in the hatch, and that they are very fearful of something or someone outside of the hatch (disease, out of control “monster”, etc).
Therefore, the procedure for retrieving the supplies involves designating one person to venture out and fetch the food, and anyone else gathers inside the lockdown room. Only once the lockdown is in place does the food fetcher go out and retrieve the food, and the blast doors remain sealed until the food person comes back in, examines everything and determines that the outer areas of the hatch are safe after the external doors have been re-sealed. Only then are the blast doors reopened, so that if anything did get in while the hatch was open to the outside world, the person/people in the lockdown area would still be safe.
Just another of my crazy ideas; hope it works out.
I like your theory. Sounds very reasonable.