One commonality between the spaces that exist in the back rooms are ones that emulate doctors offices, malls, corridors, pools, playgrounds, etc. all of these are either public or pseudo public spaces (like big box stores). Also there is a nostalgic quality to them, they all exist in a “built in the past” sort of context.
I’ve thought about this a bit but not super in-depth. I think you’re right about the fear and anxiety about public spaces. I would also throw in some other theories, such as the Freudian uncanny, where something is off about a familiar space that causes dread, and a desire to correct what’s off-putting. Imagine ANY contained space that you’re unable to escape; surely any bland office floor you’re on must have an exit and a staircase–now imagine that it doesn’t. Another writer, Roland Barthes, talked about young people really enjoying building forts and being in boats, what I’ve seen elsewhere described as “amniotic tranquility,” where something is storming or thrashing outside of your contained, safe space. The backrooms seems like an inversion of amniotic tranquility, where the protective fort is suddenly what you need to escape from. Hence the fascination on some subconscious level.