Text box text reflows when whole box is copied

Can anyone explain how text can reflow simply when copying a text box and pasting it? See screenshots. Maybe there's some default paragraph setting someplace that's causing this, but I couldn't suss it out. I'm not resizing the box at all. Just selecting, copying, pasting. Also happens when I use the Move command and hit copy—and not moving it at all, actually, just letting it paste on top of itself.

I need a genius . . .