I will say that there can be, depending on the culture (or sub-culture), a tremendous amount of pressure on brides to have a "perfect" wedding. Most of us are told from childhood, in direct and indirect ways, that this is the biggest day of our lives, the height of our personal beauty, the most important party we'll ever throw, the photos will be up on our mantels forever, etc. etc. etc.... And then you add to this everyone telling you that it's your day and should happen your way, and it's a recipe for disaster. I remember when I first started planning mine, I was having near-daily tear-filled breakdowns over tiny details just due to the stress. I don't think I did anything particularly bridezilla-ish during that time, but I can relate to the impulse. Within a few months, though, as big decisions came together and I shed some of that wedding-industry baggage, I did start to feel less overwhelmed and my emotional stability improved. That's not to say that I think anyone should just excuse bratty behavior, but I do get where it comes from and I think it's good to cut the bride some slack once in a while. I see the term "bridezilla" thrown around a lot and I don't always think it's warranted.
If she's got that kind of wedding stress and it's colliding with her personal stress, I can definitely see it manifesting in some pretty untoward behavior. It sounds to me like your best bet is to choose your battles (and do stand up for yourself!) and otherwise just ride things out to see if she improves. If she really gets to a point where she's starting to alienate people she cares about, it might be a good idea to pull her aside and point it out to her - maybe in the interest of seeing if there's some way you can help alleviate some stress.