App store sign-in keeps popping up

I'm on a 2014 Mac Air running up-to-date El Capitan.

I haven't bought anything from the app store lately (trying to remember if I have at all for this particular computer - I don't think so.)

The app store login prompt keeps popping up while I'm working. I don't have anything open that would make that likely or necessary - not iTunes, nor anything else.

But it's popping up two or three times a day.

What's happening, and how can I make it stop?

Speak slowly, I'm not very tech-savvy.😉

Thanks!