This phenomena is not a fluke, and it apparently just happened again.
Today I imported a new batch of subscribers to one of my lists with a CSV file. Immediately before doing this, I unpaused my Welcome autoresponder that I use specifically for Booksweeps imports.
I imported the batch.
When it appeared to be finished sending, I paused that autoresponder again.
A few minutes later, my wife (who's been subscribed to both my lists for a long time) tells me, "Hey, I just got this weird Booksweeps sign up welcome email from your newsletter."
I have no idea what is going on, but random autoresponder emails going out is not okay.