Why would non-drive side leading spokes break? I've built about half a dozen wheels for my commuter bikes and never had any problems, but the last rear wheel broke some spokes.

Everything on the wheel was brand new, 32 spokes, had only done about 15 miles. I was pedaling hard, probably accelerating from 15mph up to 25 mph, and I broke a few spokes. I checked the wheel and it was okay to pedal slowly to my office as it was only a few minutes away, and I leaned on the bars to keep as much weight off the rear wheel as I could. Ended up breaking six (!) spokes by the time I got there.

All but one broke at the rim, and all but one were non-drive side leading spokes (i.e. the last set of spokes to be installed as per https://www.sheldonbrown.com/wheelbuild.html#leading).

There was obviously something wrong, maybe too much/too little tension, or a tension imbalance somewhere, but what could have caused just one set of spokes to fail so greatly? I would have expected drive side and trailing spokes to break first under stress.

Just so I don't make the same mistake again. In retrospect I should have checked the tension on the remaining spokes after the first two broke, that's what I did in the evening to get me home six spokes down and it was still fairly solid.
Spokes generally break when they are not tensioned enough, because they can then cycle from being taut and being loose as the wheel rolls, and because of this they fatigue and fail. Specifically, as the wheel rolls on the ground, when a spoke is pointing up, away from the ground, it does not carry any of the bike and rider weight and is tensioned, but when it is pointing down, it carries weight and this counteracts its tension, so an insufficiently tensioned spoke will lose all its tension and become loose. This taught-loose fatigue cycle repeats every wheel turn, which quickly adds up to a lot of cycles and breaks the spoke. As the non-drive side spokes of the rear wheel are less tensioned to begin with, it makes sense for them to be more prone to this issue, and the point at which a spoke breaks is usually at the rim, because that is the location of the spoke threads, which cause stress concentraction (they can be thought of as little cuts in the otherwise smooth spoke surface).
I have seen same wheels, one properly tensioned and one not (easily checked by squeezing the spokes together by hand), behave completely differently becasue of this. The tensioned wheel was virtually undestructible, whereas the loose wheel had its spokes break all the time. The owner of the loose wheel thought his wheel was junk, when in fact all he needed to do is tension its spokes properly. The moral is, when you buy wheels, check that they have been tensioned properly out of the factory, it makes a world of difference.