Those snakes were totally having sex!
I got in touch with a Buffalo-area herp club and they said, yes, garter snakes do sometimes mate in the fall. The female can store sperm over the winter, and if she isn’t fertilized again in the spring, she’s ready to produce her little snakelets later in the summer. How cool is that?
Moral of the story: ALWAYS bring your camera — you don’t know what amazing thing you’re going to see!