I'm finding that when I'm using Krita, the colour picker/brush settings popup will often appear of its own accord. This only seems to happen when I'm using the eraser on my tablet pen, and is a bit inconsistent in terms of when it decides to appear. Nonetheless, it's really annoying because I keep accidentally picking different colours and having to break my flow to go and re-pick the one I was using.

What shortcut is this popup tied to, and can I turn it off?

By default that pop-up palette is assigned to the right-click button. Some people accidentally press that without knowing. Not sure if that is what is happening to you.

Oh that's interesting - it could be possible that I'm brushing one of the buttons on the side of the pen specifically when I've got it upside down. I've found the setting and disabled it for now, I'll see if it makes any difference.