It wasn't just one model that was broken--though the one you are thinking of was particularly bad--but of course it is a combination of bad design and a defect... what else could it be?... the batteries somehow revolting?... a coordinated cyberattack?... like, obviously it is bad design and a manufacturing defect ;P.

Here is a video that went into a lot of analysis on the wider-scale issues of Samsung's low-quality batteries.

https://www.youtube.com/watch?v=OfM0GqsIB6c