gameplay bug

Vi's ultimate is meant to be unstoppable yes? no spell in the game can end its channel before it hits target? well in game just before as i flew past volibear towards a miss fortune with my r, his q threw me right over his shoulder, cancelling my ultimate and putting it back on cd. i dont know if this is intended, however i doubt it, this q meant mf lived and we lost the game shortly after as our team was aced when i couldnt destroy their backline. Is this intended or a random error and i am just super unclucky?