I am going to make a guess here and some may not like it.
I suspect that the oldest son inherited the family farm and/or herds of livestock, and the younger sons had no other way to earn a living except to become mercenaries.
A conflict of this size usually involves Kingship.
I believe what we are seeing here is a large battle involving royals and their forces, many of whom were mercenaries from all across Europe.
The welferites run out of money and aksed richies to share.
Richies said go getta job......aaaand heads were cracked.
Remember like it was today.