I don't know what year that was, but in the first half of the 20th century it was not uncommon for troops to engage the slow, low-flying early aircraft that might appear over their trenches. Sure wouldn't be hard to bring down a early Sopwith, SPAD, or Fokker with a well-placed .30 cal round.
Especially in volley fire, as shown.
I believe more than a few rotary-wing craft crashes as late as Vietnam were attributed to small arms fire -- AKs, SKSs, Mosins and the like being the most likely culprits.