I thought: has my account been hacked? Did all the people who were following just all get tired of my nonsense and stop following me all at the same time?
No, it turns out that my third guess was the right one - sort of. It was the result of Twitter resetting, after a bug had been exploited which allowed one to force someone to follow them.
Venture Beat puts it’s better than I can:
“All you had to do was tweet “accept” followed by the person’s Twitter handle with the “@” sign included, and they would immediately follow you.”
And a well-known example of this would be “accept @jesus”
Anyway, it should all be fixed by the time I log in today.
[Update]: Yes, it is fixed.
Source: Twitter temporarily hits the reset button on follower counts amid bugs [Venture Beat]