Thunderbird does not automatically archive messages. https://support.mozilla.org/kb/archived-messages

If messages get archived automatically, then this must be triggered elsewhere, e.g. on your email provider's server.

Thanks for your comments christ1, but I don't see where the server would be sending copies to the Thunderbird archive. I have two gmail addresses, and it only does it for one of them. The only difference in their settings is that the one that works normally is set up as a POP server, the archiving one as IMAP.

I don't see where the server would be sending copies to the Thunderbird archive.

Your account is IMAP. With IMAP, all mail is managed on the server, and synced to clients connecting to the server. I don't know what the server is doing, but it wouldn't be impossible that the server triggers some sort of archiving action. I have no other rational explanation.