Several financial institutions are following up immediately if a payment is not received by the due date. Some calls are friendly reminders. Other calls are more aggressive in nature.
CapOne's roots are in subprime cards and while they've gone more mainstream in some areas, usually by acquiring banks with branch networks they rebrand Capital One (Texas 4 years ago, now Chevy Chase Bank in the DC area), they are always going to be aggressive about any delinquency, and are very quick to sue. They don't want to wait 30 days - if you're late they want to hear the right things when they call or you may find your account closed instantly. This may also be ratcheting up with the changes in regulations so they aren't as able to instantly hit you with massive fees and punitive interest rates.
It's almost certainly their in-house collections department that's calling. We all have to be extra vigilant, cannot ever be so much as a day late!