Whenever I talk to bike shops about wheels, they always trash-talk the "inferior" stock wheels that come with bikes. "Oh, no wonder your wheel is out of true, you're using a stock wheel", etc...
There is probably a mixture of truth and bike mechanic ego / elitism there, as well as their desire to up-sell me on more expensive components. Stock wheels have generally been sufficient for my bicycle riding life. But I was worried about the 28-spoke-count for this wheel, being lower than all my other wheels I've used, and the resulting weakness mentioned by cyccommute.

However, the overwhelming consensus seems to be that this is probably due to a manufacturing defect of the wheel since these breakages are definitely unusual.

I bought the bike online, from Jenson USA, so it's not local for me to take it back directly. A cursory search online seems to indicate that everything is covered by the part manufacturer's warranty (in this case, WTB), which should still apply. Convincing them it is a manufacturing defect and not something else (e.g., abuse by user) might or might not be a challenge, I don't know. As also mentioned by cyccommute, any successful warranty claim would likely just be a replacement wheel of the same type...though presumably one without whatever defect is causing this issue. A replacement would be fine with me but I do intend to take this bike touring with me and carrying much heavier loads than my weekly grocery trips, and I am foreseeing problems if I stick even a properly build replacement wheel.

Hahaha, this made me laugh.