Why would non-drive side leading spokes break?

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.