A product that is worn on one’s head potentially exploding or spontaneously causing a chemical fire due to to suboptimal environmental factors counts as a design flaw, at least in my book. Something that is used on people’s heads should not fail that way if it is going fail.
That depends on how it was treated right up to that point. For instance: unauthorized repair, wrong substitution parts, messing with the charging circuitry and so on are things that can and do happen.
It need not be a design flaw right away, but it well may be. Of course you could be of the opinion that it should be impossible for something like this to happen, and that is definitely a valid position. But even wired headphones have on occasion killed people (a Swiss HAM, for instance).
