Bicycle Mechanics - I give... spoke/wheel question

I did do a search, I give up...
I broke a non-drive side spoke at 250 miles on a new rear wheel for my single speed. So, I pulled the wheelset and plugged in a set of Rolf Vectors that I've had sitting in the closet for about a year, and set it up for my single speed. On my commute to work today at 15 miles I heard that heart sinking thawng, and found a non-drive side spoke had snapped at the hub, just like the new wheelset.
So, there is either something with the wheels I'm using, or something with the bike, an old large tube Cannondale. Is it possible that in creating this single speed I didn't take into account the torque that is put on the rear wheel, even though I've spaced the gear into the chain line with spacers. I'm running a 15 cog on the rear with a 53 up front and 175 cranks. Not only that, but that the Cannondale tends to be a stiffer frame and that stiffness is adding to the torque put on the wheel.
I have several other bikes and wheelsets and none have had this happen, so I have to ask. It's frustrating, as this is my fave ride.

Sounds like a freak occurence. Odd that the non drive is breaking as they have less tension(typically) than the drive. Where did they break, what kind of hub on the single speed, what brand spokes and are they black anodised (These seem to fail more often)

Reduced-spoke wheels all suffer from the same fundamental engineering blunder, in the sense that the strongest, most reliable wheel, with the best strength-to-weight ratio, is the one with the most, not the fewest, spokes. If you are racing, want to minimize turbulence-induced aerodynamic drag, and buy new equipment every year, by all means use reduced-spoke wheels. However, if you are John Q. Public interested in a reliable machine for commuting or recreation, use wheels with at least 32 spokes each. I think your two occurrences of nondrive spoke breakage are simply coincidence, although inadequate overall spoke tension will promote nondrive side breakage.