Event registration expires - why?
The message is a little misleading. This email is generated if you've got the field "Pending participant expiration (hours)" set on the "Online Registration" tab of your event configuration. The help text for this field states:


  Time limit in hours for confirming/finishing registration by
  participants with any of the pending statuses. Enter 0 (or leave
  empty) to disable this feature.


If you don't have participant approval or waitlisting enabled for your event, then the likely cause is that your PayPal transactions are getting stuck in a pending state. You've probably seen the "Pending (Incomplete Transaction)" status on contributions in your database. Have a look in PayPal to see if the funds are actually going through. If they are, you probably have a config problem keeping PayPal from pinging CiviCRM to let it know that the transaction was successful. Is your notification URL set in PayPal?