Hello,
MYT by <null>via synchronized can be caused due to scripts or any event-options triggering the commit and I'm not sure about the other.
The best way to investigate this behaviour is to check in the interactive-logs. If you haven't configured interactive-logs, please find the KB article for the procedure - https://kb.juniper.net/InfoCenter/index?page=content&id=KB30458. This way we can correlate the timestamps and can check which has triggered the commit operation.