I think an (not: the most) important reason for the crusades was exactly that: to give non-first born nobles a good opportunity to carve out a fiefdom elsewhere. Check out the various crusader counties and principalities. You'll see that the vast majority were set up by second/third/etc sons of noblemen.
Wikipedia about Godfrey
Godfrey of Bouillon was born around 1060 as the second son of Eustace II, Count of Boulogne, and Ida, daughter of the Lotharingian duke Godfrey the Bearded by his first wife, Doda.
Wikipedia about Baldwin
Baldwin I, also known as Baldwin of Boulogne (1060s – 2 April 1118), was the first count of Edessa from 1098 to 1100, and the second crusader ruler and first King of Jerusalem from 1100 to his death. Being a younger son, he was destined for a Church career, but he abandoned it and married a Norman noblewoman,
Two examples of non-first borns moving up the medieval career ladder. See it as a win-win situation for everyone. The first born didn't have to worry a lot about his younger brothers snuffing him out. The neighbors didn't have to worry for that nasty pest next door pilfering their lands. The younger brothers got what they wanted and otherwise never would get.