When you update the owner of a post, it updates the timestamp for that topic.
This was an expected behavior that felt like a bug. If this is not a bug, I suggest we add an option to the change ownership modal dialog to leave the timestamp as-is.
We imported a lot of historical conversations into our Discourse instance. Because of the platform we came from, that included a bunch of duplicate user accounts. As part of a general cleanup, we were de-duping those user accounts and changed ownership of posts. That pushed old topics up to the front page. We had to go back and re-adjust the timestamps for all these topics back to get them back in their expected chronological order.