Why do cyclists call out "door" when going around your door as you're trying to get out of the car?

It's happened to me twice now, and I don't know why they're doing it. Is it to let other road users (cars) know the reason they're changing traffic lanes? I find it tough to believe they're gonna be heard by a car dozens of metres away or zipping by (presumably they're not switching lanes into a car so there's at least some gap between us and the nearest car).

Or is it for my situational awareness that I have a door open? I just don't know why that would be it, presumably there's no way for them to miss that there's no way for me to miss that my door is open.

Hoping to hear from some cyclists or other road users who know this dark art. Once was odd, but twice tells me there's a practice about this and doubtlessly a rationale behind it.

Thanks!