What could be reason that my anal herpes are flaring up again?

Triggers?: Triggers for herpes can include illness, stress, fatigue, becoming physically cold, trauma or one's period.

Probably random: Genital and anal HSV2 outbreaks seem to be random. No valid research has identifed triggers -- unlike oral HSV1, with outbreaks often triggered by injury (sunburn, surgery) or other infections (hence names "cold sore", "fever blister"). Some patients find stress or other things can be genital/anal triggers, so if you suspect something easy to avoid, give it a try. But don't expect miracles.