I would expect it's due to the Estrogen receptors resetting effectively due to the break from estrogen, I can't say I'd want to experiment and find out though! In effect maintaining high levels for a long period of time will cause the Estrogen receptors to become less effective so the enforced break means they are hungry for Estrogen again which means breast growth.

Kind of like taking a break from alcohol or any other substance of choice the body isn't as tolerant of it as it had become.....Right now my drug of choice would be some cocodamol as just finished two hours of electrolysis and am sort of feeling in a lot of pain!