With the exception of cars, mandatory recalls are kind of a new thing. The FDA only got mandatory recall authority in 2011, and the Consumer Product Safety Commission didn't have mandatory recall authority until 2010.
As for what kind of a flaw is serious enough to warrant a mandatory recall, I don't know, but I don't think it's as simple as meeting a threshold for the percentage of affected products.
If a particular flaw is near-universal in a company's product, but no one is likely to be hurt as a result, then there won't be a mandatory recall. But a demonstrated flaw that has the potential to maim/kill someone will probably generate a recall, no matter how few units are affected.