Similiar to the IPO where the stock could not go below IPO price today it can not go below $19.
Primary reason is because banks stand to lose money if they let it go below $19. Retail is primary purchasers of options and Big Banks/Market Markers are the primary sellers of options.
If they allowed this stock to go below $19 they would lose money and the government wouldnt allow that. Therefore it does crack below $19 until next week when they can rinse and repeat. Spike up the premiums, suck in the volume then pin the price to whatever price they so desire.
One billion shares of FB could trade today and this stock would still not close below $19. Big banks control every stock and were it closes on option expiration day based on where they make the most money. Even if they do lose they are backed by the government printing press bail out system.