A fire at a Samsung plant actively involved in recycling Note 7 batteries isn't good, but it can be spun in an encouraging way. First, it can be argued to represent objective proof that no, these batteries were dangerous and Samsung's recall was the right thing to do. Perhaps more importantly, investors don't really care if the parts of your plant involved in waste management and recycling catch fire (not unless you're a recycling plant, anyway, in which case that's bad).
Acknowledging that a fire began in the area of the plant dedicated to producing batteries for upcoming Samsung products, on the other hand… well. There's this little device you've probably heard of, called the Galaxy S8, and Samsung is hoping it'll prove very popular, washing out the melted-phone-and-charred-house odor of the Galaxy Note 7. A fire at one of its facilities dedicated to producing smartphone batteries in particular, at a subsidiary already blamed for the Note 7 recall is a bad way to accomplish that goal.
There is as yet no sign that Samsung's Note 7 fiasco has hit the company's popularity or appeal, but that's the kind of problem you don't keep provoking. Having screwed the pooch on the Note 7, Samsung will be dead-set on ensuring the same thing doesn't happen to the Galaxy S8. One bad product is a fluke. Two are typically a sign that something is badly wrong in your manufacturing division.