The greatest predictor of future weight gain is past weight loss. If you try to cut weight that quickly, you're likely to be a part of that statistic.
If you're losing a significant amount of weight over a short period of time, chances are you're losing muscle instead of, or as well as, fat. This might be a contributor to the above mentioned weight gain predictor. If you cut a bunch of weight, but it's mostly muscle mass, when you gain that muscle mass back, you'll also gain some fat with it, and suddenly you're heavier than before.
Rapid weight loss can also be water loss, which is to say, dehydration. That's not good for you either.
Really, if your goal is to lose fat, which if you have a ton of it, isn't a bad idea (if you're just in the 'overweight' category, I wouldn't worry about it unless you actually feel unhealthy), I would shake any notion of "weight loss". I would replace it with a goal of losing inches, and reducing body fat percentage. This allows for weight gain, due to added muscle, while losing fat. That's more likely to be sustainable. Muscle loss isn't sustainable - you'll eventually crash and have to build it back up (or be perpetually unhealthy; another likely contributor to the negative health effects associated with being overweight or obese is unhealthy weight loss practices).