Internet and the flood of information. Vegan population soon reaching a critical mass(every friend circle has a vegan). And then when people have someone next to them who is vegan for x+ years and healthy they will think it's a maintainable lifestyle.
Also, anecdotal evidence, just a year ago a vegan post on a forum (i.e. Reddit) would get downvoted into oblivion and posts like 'bacon tho' upvoted, when today, more often than not, it's the opposite