There are many many reasons why a full redownload of thousands of email can occur. Few of them have anything to do with Thunderbird, other than the observed symptom, although a compact on an IMAP acocunt is one user initiated action that can involve Thunderbird.

It becomes noteworthy if it keeps repeating after it has completed the process. Note that turning the computer off while it is in process, or force quitting using the operating system will generally see the process start at the beginning on the next start because the details of the download are updated when it finalizes, not after each email gets to you.

Thanks much for your info. If you are still available, what is a "compact on an IMAP account"? I'm concerned it might be a hacker trying to get more info about me, since I did have a security breach recently.