1. They were all visa cards. Visa is releasing their new "chip and pin" business and if those people's cards were expiring before they could get the new technology cards, they might have short expiry dates on their cards they got, to force the upgrade to the new cards.
2. They all came from the same "printer" and there was a screw up, stamping them with the wrong date.
My bet would be thought number 1, as I know I got a mail from my bank last month about the "new" cards, although mine doesn't expire until 09/09 anyway.