There are lots of things that Time Machine does to maintain the backup data set. If you have it set to run automatically other than occasional extended spurts of activity on the Time Machine disk to is unlikely you would ever notice their taking place. These include:
- the backup of every file that has changed in the past hour
- the consolidation of hourly backups to daily backups including weeding out the extra hourly backups of files that were changed more than once in the day
- the consolidation of daily backups into weekly backups including weeding out files that were backed up more than once during the week
- the consolidation of weekly backups into bi-weekly backups including weeding out coies of files that were changed more than once during the period
- the periodic checking of the entire backup set to assure no corruption or error has managed to work its way into the backup data set.
When Time Machine is backing up automatically it runs every hour and every run will include the hourly backup, one run each day will include the hourly backup plus the daily consolidation, one run a week will include the weekly consolidation and one every two weeks will include the weekly and bi-weekly consolidations. I don't know how often the backup verification run is performed but I suspect it is at least daily.
When you are backing up manually I don't know what triggers the various consolidation routines and the verification routine but I suspect it is triggered by the calendar and whether or not the particular consolidation run has been performed in the interim. Basically you are defeating the purpose of Time Machine but it has to have some fall back mechanism to compensate. If you are really curious you might try the
Developer Documentation. If you want more control take a look at
this OWC Blog (although they may not work with Mojave) and neither provides a mechanism to control when the various consolidations or the verification occurs.