Let me tell a personal anecdote. Feel free to dismiss both this and my conclusion as statistically irrelevant.
When my son was a baby, like all babies there came a time when he started reaching for and grabbing things put in front of him. He was doing this invariably with his right arm and hand. Which I considered a good thing, since being a realist I accept that left-handedness would be a handicap to anyone wanting to use tools. Since most are designed for right-handers.
Then one of his scheduled immunization shots came due. We took him to the local clinic, I was there when the lady gave him the shot. In his upper right arm.
He never, and I mean NEVER used his right arm to reach for things and grab them, ever again. It was extremely, glaringly obvious. From that day he grew up left handed. His handwriting is typical left handed (a handicap in itself) and he's awkward with tools.
It turns out there's a general tendency for medical people to give injections in the right arm, to babies (who can't express any preference.) More convenient for the doctors, since they are predominantly right-handed.
There has been a dramatic increase in left-handedness in Western countries over the last 50+years.
From that experience I would bet it's related to immunizations being given to babies in their right arm, at a time critical to formation of left-right brain wiring. And the effect becomes permanent.
Whether it's just a result of avoidance of using a sore arm during that critical period, or perhaps more serious local nerve damage (don't dismiss that, there's the whole issue of thimerosal in vaccines, a preservative/adjuvant based on mercury, which is unarguably a neurotoxin), I can't say.
But then there's things like this:
http://scitechconnect.elsevier.com/rates-of-left-handedness-downs-and-ups/(Graph attached below.)
Would you trust that graph? I don't. I'd suspect it's formatted to hide that the last bar is a lie, and recent/current left-handedness numbers are much higher than 12%. But we can't be showing precipitous rises now can we, or people might wonder about causes.