He survived a lynching
That is not just speculation, that is in the script:
EXT - SOMEWHERE IN ENGLAND - DAY
A bunch of SOLDIERS are lined up at attention.
LIEUTENANT ALDO.RAINE, a hillbilly from the mountains of
Tennessee, walks down the line. He recruits the men, the
Germans will later call; "The Basterds". Lt.Aldo has one
defining physical characteristic, a ROPE BURN around his
neck. As if once upon a time,. he survived a LYNCHING.
The scar will never once be mentioned.
My name is Lt.Aldo Raine, and I'm
puttin together a special team.
That may be because he is part American Indian
At 1:20, he says
And I am the direct descendent of the mountain man, Jim Bridger.
That means I got a little Injun‡ in me.
An implication of this might be that an attempt to lynch him was made out of racist motivations, but that is speculation.
‡ Injun: Indian, or Native American.