If its authentic its a pretty amazing shot considering the fact that cameras weren’t nearly as prevalent then as they are now.
Indeed.

Robert Capa
It’s easily explained. The soldier was sniped when he stood to get his photo taken.
If that mass on top of his head is what I think it is, it is further proof of sniping, suggesting the round came low from the other side of the hill that drops off to the soldier’s left.
It seems most likely the photo is genuine:
http://www.pbs.org/wnet/americanmasters/episodes/robert-capa/in-love-and-war/47/
According to the above link, the fallen soldier’s name was Federico Borrell García, and he was killed in battle on September 5, 1936.
I know that a camera was filming and got the same shot but to get it the instant of the highest drama is amazing.
Even with the scarcity of cameras back in the 1930s, the majority of photographs taken back then were taken with cameras.