There are many possibilities for this.
Were you more stressed? Tired? Were you getting enough sleep? Have a secondary infection? (You don't mention your gender, but if you have a vagina, it's possible that you might have a yeast infection or bacterial vaginitis, or another STD. If you have a penis, it could be a fungal infection or another STD.) Have you been sick a lot? If you have a cold, the flu - things like that, your body is trying to fight one thing, making it harder to fight others.
Hormonal issues can also trigger outbreaks - menstruation, birth control pills, menopause, etc. Also, medical conditions that affect hormones - thyroid problems, for example - and some medications can cause this as well.
If you shave your genital region, that might be triggering the outbreaks. Some people find using a trimmer helps, or if you are using a trimmer, try a razor or waxing. Changing your hair removal method may help.
You may not ever figure out what is causing the increase, but consider suppressive therapy, like daily valtrex or acyclovir. Valtrex is more expensive, but is a convenient once a day dose. Acyclovir is MUCH cheaper, but it's twice a day. If you are in the US, your choice may depend on cost/insurance coverage.