I'm another who is skeptical of the sweat theory.

I tend to first think of improper maintenance/repair in cases like this. One possibility that comes to mind is that at some point a mechanic/owner attempted to adjust steering bearing preload by torquing the cap screw. They felt some play in the steering head and started wrenching down hard on the cap screw which does nothing except compress the stem clamp area unless the clamp screws are first loosened. I wonder if this might cause a localized fracture near the top as the cap tries to compress the stem clamp which, over time, would start a crack which would propagate down the stem clamp area. And as the crack progresses, it would be over-stressing the other side as well which could cause a corresponding crack. It's just a theory, but I think it has some merit if there is any possibility the bike has been worked on by someone who doesn't understand how threadless stems work.

If not this, then I tend to think next of crash damage, particularly a very hard hit on the handlebars.

Final possibility is simply a defect in the product. I'd definitely get in contact with the mfg and I suspect they'll want to see the part. They'll probably be happy to provide a new replacement. Unless this stem has a bad reputation, I'd chalk it off as a one-off failure.

Glad you're Okay.

- Mark