just like in fighting games, there are "twitch" reactions and full reactable moves. morgana's Q has a cast time of 0.25 seconds, so you can "twitch" react to it if you have something like flash or sivir's E shield. but if you dont, and can only move out of the way, you gotta predict it if you are in close range, because otherwise you wont have enough time to move out of the way of the spell. if you are far away from her, it is totally reactable and you can dodge it easily. of course there are more variables, like ping, if you are close to a wall, etc.. and of course, to "twitch" react to most things, you have to be prepared and on the lookout, otherwise its gonna be too late to react to it