Usual story, someone evaded security, passengers sent back from boarding gates to be re-screened, planes de-boarded, so passengers could be re-screened. And with the hubbing model airlines use any delays will ripple out through the wider air transport system.
This has happened 3 or 4 times that I can remember. I wonder how many such breaches are undetected?
Might be time to beef up the security to prevent breaches?