As I drove along, I realized what was wrong. My primary credit card was reissued a few days ago with a new number for security reasons. That's the card to which toll replenishments are billed. The balance in my account had dipped below the replinishment amount but the first attempt to recharge it had failed on account of the card number on the account was no longer valid.
When I got to the office, I logged in to my FasTrak account and fixed the billing. There was a backup billing, so I wouldn't have been totally hosed, but I was down to only one more trip across one of the Bay Area bridges.