Cycle touring. First chain failure. Thoughts on cause? Was pedalling up steep hill on gravel road. No gear change. Just seemed to go mid push. Was very steep and out of saddle....had to push the remainder of way!
Well, only really one proximate cause for this: chain was weakened through wear and fatigue (it looks a bit old), you applied enough force to break it. 

Likely that there is no single event that broke or weakened the chain. 

Chain failure happens. Think of it a a rite of passage.