Why do girls flirt with you a month before they move somewhere far away?

I am platonic friend with a lot of girls and I'm usually very casual, maintaining boundaries with all of them. Dozens of girls come and go into my life all the time like a revolving door, and what I've noticed is that in most case about 1-2 months before the girl is about to leave my life (leave school, job, community, state, country) (they don't tell me directly but I know they are about to leave from my network) the girls always start flirting aggressively with me whereas before we talk casually, the girls start getting very close physically, making sexual jokes, ask me out, etc...