Its a sign of maturity and that they are now well able to start flowering. You can veg a photo-dependant plant for ever if you want, but most are ready to flower from seed after a couple months. Auto flowering plants will flower on their own even with 24 hour light schedule at a pre-determined time due to genetics. your harvest is set I stone during veg so let it grow as big as you can house it before flipping. Hope this helps. Cheers