Why is a number of notifications showing up on one of my calendars?
I discovered the cause once I arrived at work: the notifications, which came from an airline, were sitting on my desktop computer, waiting to be approved. For some reason, they didn't pop up on my laptop while I was at home. (possibly different OS version: 10.11.6 vs. 10.10.5)

It still isn't disappearing from my laptop, but hopefully if I wait it out a while Apple's Calendar will get around to noticing.