One reason: "It was believed among the English archers during the Hundred Years War that the French intended to cut off the first and second right hand fingers of every captured archer to prevent him from again using a bow. The archers raised those two fingers to the advancing French as a gesture of defiance." Brittishbattles.com

Same reason, .org site: "The story is that French soldiers at Agincourt threatened to cut off the index and middle fingers of captured archers in the army of Henry V. Actually the archers were almost all Welsh - not English. The Welsh had invented the longbow, at that time the most feared weapon in Europe (so much so that it was widely assumed there could never be another war as the army with the longbow would always win - plus ca change eh?). Anyway, after crushing defeats at Poitiers and Crecy, the French army - still without Joan D'Arc who wasn't to rescue their fortunes for several years (this was the Hundred Years War, remember) - assembled at Agincourt in 1415 thinking they had the upper hand at last. Their confident troops (they outnumbered Henry's army by about seven to one) approached the English camp (battles were decided quite formally in those days) made the boast that they would cut off the two fingers of any captured archers and then returned to their camp." oii.org