☝️
Accidentally hitting the O key (instead of 0, for instance), or just forgetting that you had Proportional Editing enabled, is something that has probably tripped all of us up at some point (possibly even more than once😉).
And also, like Wayne said, Kudos for including the video!
Thanks! It took me a while but I realized what I'd done. That was a real facepalm moment. :)
edit: I never would have expected proportional editing to affect the other objects that weren't selected at the time in the scene either. Learned something new there.