Oil Drain Plug

We have our oil changed by a local dealer (will with hold name out of courtesy to them). They have been performing changes regularly over the last 3 years. Today, they informed me that the drain plug was stripped sufficiently enough to leak and that the oil pan needed replacement in order to fix it. Is this a problem due to their negligence or is this a common occurrence? I was always taught (and the manual states) to replace the washer (if necessary), hand tighten the drain plug after cleaning, then turn with wrench 1/4 to 3/4 of a turn. But do not over tighten! Could the stripping and subsequent need for replacement be caused by any other thing than their negligence and over tightening of the drain plug? Need some advice here on my next plan of action.