Horizontal (rear facing) drop-outs and braking

I've never used a bike with rear facing dropouts but I always assumed that accidentally ejecting a wheel from them would be an extremely difficult thing to do. My simplistic brain struggles to come up with a scenario where it could happen, the only thing that I can think of is that your skewer was looser than you thought and/or the wheel was not slid far enough into the dropout so the skewer didn't have enough dropout material to grip reliably.

I guess another possibility is that some lube, or more likely grit, got between the skewer/axle and dropout faces when you fitted the wheel, again compromising the grip. A tight skewer should overcome contamination by lube though, I'd expect. (I can think of no way to word that last sentence without it sounding stupidly smutty, so I'll just leave it there as it is!).