COI expiration happens when one or more policies on a certificate reach their expiration date without a renewal on file. The sub keeps working, but the certificate is now stale and the GC's proof of coverage is gone.
Because expirations are easy to miss, especially when different coverages renew on different dates, automated tracking with reminders at 90, 60, 30, and 7 days is the most reliable defense against a coverage gap nobody noticed.