System Account deleted site collection

First off, the "System Account" is typically used by Microsoft's internal services and processes, so it shouldn't be initiating deletions on its own accord. This could potentially indicate some kind of issue or bug within SharePoint Online itself. Before proceeding further, I'd recommend checking the Deleted Sites utility in the SharePoint admin center. Look there first to see if you can restore any of the deleted site collections. The sooner you catch them there, the better chances of recovery.

Moving forward, you'll want to dig into what's triggering these deletions. One avenue to explore is checking the audit log reports in the compliance center. Look for any entries around the times the sites were deleted and see if any patterns emerge around the "System Account" activity. Another possibility could be some kind of app, workflow, or script running with elevated permissions that's inadvertently causing the deletions. Review any third-party solutions integrated with your SharePoint environment.