Here is a complete guess....
I assume you are male. From what I gather, your last step father began to physically abuse you around the age of 10.
At 15, you were well into puberty and growing. You were working with him- “Man’s Work” I assume, and he probably:
1) noticed the changes in your body.
2) realized the burgeoning threat to his physical dominance
3) Wanted to test you
4) Wanted to re-assert some form of Dominance
It may also have been, in a way, some sort of reprimand for physically maturing enough so you could defend yourself.
My best guess. Hope it helps.