It's called "multi-sourcing"-- ensures sufficient supply and keeps prices in check by playing off one supplier against other. Every OEM computer manufacturer does it to increase (maintain) margins.
Not in the case of screens its not, the reason was supply, it was acknowledged a while ago that supply was a problem hence the new suppliers.
It was the same for the iPad screen too..read here..
In July, LG's CEO Kwon Young-soo told reporters that they were simply unable to meet the production demands for the iPad:
"Demand (from Apple) keeps growing and we can't meet it all. Apple may have to delay launches of the iPad for some countries due to tight component supplies and strong demand. We are considering increasing production lines for iPad products but overall supply is likely to remain tight until early next year."