It's awful to hear about, it's an ever-present danger with barns. It could have been electrical, could have been spontaneous combustion with the hay, a cigarette in the shavings, you just don't know til after the fire marshal completes his investigation.
The barn where I rode as a kid burned down in the 60s. It turned out to have been electrical. 40 horses died.
I’m so sorry to hear about the loss of the horses, although releived that it wasn’t your barn. My biggest fear I think and the reason every horse in my barn has an outside exit and the only time they are closed is if I have a mare foaling or a show date the next day.