It's hard to say what could of really happened... I worked As a tech for ford and the only time I have EVER seen a drain plug or oil pan thread strip, was when the tech was not paying attention and cross threaded it, or over tightened. The worst and most stupid thing I have ever seen was a "tech" putting a 3/8 air ratchet on the drain plug to tighten it... Obviously the this stripped the pan... Anyway, I don't think it's common for these things to happen I just think it was lack of experience on the techs behalf or lack of attention while changing the oil...
Just my $.02
2014 M5 Comp Package.