|Flushing out the anti-freeze|
Next day it worked fine. I love it when things heal themselves overnight!
Anyway after a little more reading on this (turns out that owner's manual is a pretty useful reference), I think the problem was that air entered the water line when I filled the hot water tank. In the future, per the owner's manual, I will be sure to leave a hot water faucet open when filling the heater to expel the air in the tank as it fills.
Also, there are safety reasons for doing this. HowieE on Airforums says, "Firing the heater with trapped air will greatly raise the pressure and may cause a problem when you first draw water. The pressure may go high enough that the relief valves blows off while heating the tank."