There are several reasons you might see changes posted in the QuickBooks Online Audit Log as System Administration.
If you modify an entry that also impacts another entry in your data you will typically see a System Administration posting in the Audit Log. For example, if you edit a payment that was attached to an invoice, and attach it to a different invoice, you may see System Administration in reference to the the invoice associated with the changed payment. In some cases it maybe reported as an "Indirect Edit" by the User who initiates the change.
3rd Party Apps typically connect to QuickBooks Online via the System Administration credentials; therefore, the data the 3rd party App sends to QuickBooks Online will be posted as System Administration postings, or edits if the 3rd party App modifies existing data.
Scheduled activities configured within your file that take place even if you are not logged in maybe recorded as System Administration. For example, 'automatic update of bank feeds' can appear as having been created by the system.
Transaction configured as recurring are posted automatically by QuickBooks Online, therefore the transaction is recorded as having been created by System Administration.
On rare occasions, QuickBooks Online updates impacting the database as a whole may result in every transaction being 'updated' by System Administration.