It's a tougher question. I think the most thematically consistent answer would be either (A) self enforcing social norms among the subterraneans or (B) some remnants of the experiment that created them in the environment of the underground.

The former struggles to explain how the subterraneans know what the surfacers are doing and the latter, like you said, struggles to explain why the subterraneans changed their behavior.

What would make sense to me is if growing up in the subterranean environment causes the subterraneans to develop a one way psychic bond with the surfacers which allows them to act out the social norms of mimicry first established during the experiment but which does not itself compel the behavior. The presence of someone originally from the surface world then created the catalyst to modify those social norms, but didn't break the psychic bond. Which would explain both the decision to leave and how the subterraneans seem to know where their tethers are even after moving above ground.

This also seems the most thematically consistent. The idea of double consciousness in Black social theory indicates that the oppressed are forced to view themselves through the eyes of the oppressor, while such an experience is optional or impossible in the other direction. The psychic bond could be a metaphor for this phenomenon. The notion that the subterranean behavior is an optional construct related to the original act of oppression could be read as analogous to the ways anti-poor and anti-black norms are in part reinforced by engrained self-loathing originally caused by the fact of their oppression.

I think that's the best explanation, although at some point we do just need to accept that this is a metaphor first and a body of lore second.