Hi Laura,
I'm sure at this point that a case would not help. We would not be able to determine after the fact why it happened. If you have a backup database from before the accounts were deleted or if you can recreate the issue in the currrent database, then we could assist. Can those same accounts be deleted again? But to create a case to have us figure out why it happened after the fact would not be worth the effort. We have tested this and we cannot delete accounts that meet the criteria I gave you earlier.
Let me know if you have a backup from before the accounts were deleted or if you can continue to delete accounts in the database that meet the criteria above.