It's a form of BDSM if you don't address them properly they can tell you do things to them in a sexual nature that you may not like. I for one hate to be called sir. I'm not knighted and I'm not a teacher any more. But yes that's where it comes from. I hope this answers your question.
No idea why they message you, they may see you as their type or they see you as sex symbol. I really have no clue. You could ask directly. See what you get back.