What makes us subconsciously mimic the accents of others in conversation

[Igel]

8. A girlfriend from Tennessee moved to California.

Mostly lost her accent. Until she talked to somebody from home or visited home. Then she had it. Bidialectal.

I lost most of my accent (but not my grammar) when I moved from Maryland to Oregon. Now I find myself assimilating a bit to some of the phonology north of Houston--pretty generic, but with a few vowel mergers in specific contexts that drive me a bit bonkers.

The lexical part is probably just mostly priming. The rest is part of discourse pragmatics, showing good will and that you're on the same side so that the "game" of communication can be successful.

It might be interesting to see what happens if you get a group together and say that the guy with the Southern accent is an active KKK member and have the subject group be a group of Sanders progressives speaking a relatively standard variety of US English. There'd be open hostility so there'd be less conscious motivation to show good will and that you're on the same side. (Conversely, have the speakers have a NE accent and talk about having to revamp the Constitution and install a strongly socialist system with strongly RW speakers of fairly standard English as the subjects.)

Conversely, use ethnically marked varieties of English with appropriate sympathetic or hostile subject groups.