well its not theory just my assumption.....
random thought came in mind ,, every one has rivals,, more like yin and yang
like,, hashirama and madara.
naruto and obito
so i thought can sage may have some one like that i mean anti-sage..
sage wanted to bring pieace to the world,, but anti-sage wanted to distroy the world,,,'
so anti sage created jubbi,,, but lost control over it,, then sage defeted jubbi ,, your thoughts