An unfollow is not always a decision. Accounts that are suspended, deactivated or deleted also disappear from your follower list, so they can show up among your unfollowers. Before you unfollow anyone back, open the profile: if it no longer exists or is suspended, the change probably had nothing to do with you or your posts.
Because the check runs once a day, someone who follows and unfollows between two checks will not appear, and the time of an unfollow is only known to the day. Checks also need Chrome running with an x.com tab open, so no check happens on a day your computer stays off.
Every account loses some followers over time; it is a normal part of being on X. The useful signal is a pattern, such as a spike in unfollows after a certain kind of post or a change in posting rhythm, which you can cross-check against your posts in Twitter analytics.