how many times did you bust out of an mtt, bubble, not make final table, etc before you big win? probably a lot. and now, after a big win, you're experiencing variance again and you think something is going on?
one, what is happening is nothing special and happens to everyone. it's called variance and it's also the fact that you're going to bust out (not cash) in more mtts than you do cash in. it's the nature of the game and you can expect to not cash in 70%+ of the games you play.
two, if you're losing a lot after a big win, it's variance. but if you're trying to recoup those losses by playing bigger stakes, then you're just setting yourself up to fail majority of the time. just because you have a big win and your bankroll increases doesn't mean you're ready to move up and play higher stakes. especially if the idea is 'if i win at [higher buy-in], i'll get my bankroll back.'
if you're experiencing a lot of bankroll swings due to wins and losses, you might want to check your bankroll management. swings are natural, but they shouldn't be like a roller coaster with huge dips.