The use of this improper grammar most likely branched off from the contraction mine's (or mine is). For example "Your ice cream is vanilla, and mine's chocolate." Somewhere along the line, someone forgot that "is" is already part of "mine's" and they made the mistake of saying things like "Mine's is strawberry" or (Mine is is strawberry).
Eventually, the singular form of the word was completely dropped from the vocabulary, and "mine's" became used to show all ownership, even when adding "is" is inapplicable.
Edgar: "Hey Reggie, I just got my new phone today."
Reggie: "Yea, I think I'm gonna buy mines when I get my next paycheck, dawg."