There’s no rhyme or reason, it’s just a phrase people use to sound dramatic or threatening.

I'd say this is an aggressive phrase in this context. it implies that the speaker is dissatisfied and the situation is not over with future conflict pending. A variation this is "you'll be hearing from my lawyer." Another variant might be "this isn't over." Obviously the last one does not use hearing. If said in a huffy voice, it implies future conflict. You'll be hearing from me need not be threatening. If said with a smile, it could be a promise. Think of a man flirting with a woman, they exchange phone numbers. One says, "call me." The other replies, "Oh yeah, you'll be hearing from me!" with a smile. Grammatically, you raise a point, but you see the same thing with "I'll be seeing you." Same issues, context and idiom. The actor who played Moriarty was great. He and Cumberbatch played off of one another very well. Watson (Martin F) was also very good. I hope this helps. Best wishes in your studies.