There are many causes for why somebody would suddenly flare, such as a girl coming off of a birth control pill. Stressful events might be a trigger. It's very difficult to measure "does stress really make acne flare?" but anecdotally we hear that all the time so it may well be that this is a real factor.
Foods are another common but very controversial factor. Many studies over many years suggested that food has nothing to do with acne, but more recently we have an understanding that for some people, it might.
We have evidence, for example, that skimmed milk may be an aggravating factor for people who have severe acne. We also know that individuals who eat lots of fish-it may have a protective effect for those with moderate to severe acne.
There is some evidence that acne may be flared by high glycemic foods, and refined sugar. The Western diet may be a factor in producing acne in the first place. We do know that rural non-western cultures have much less acne than we do here in the West.
So there are environmental issues that we don't yet fully understand.