Pretty much title. Why is he just now seeing LCS play? As a long time Viktor main, I have understood his potential before and after the rework as a strong mid range burst mage. I have always been confused why he isn't more popular.
So why is he suddenly viable?
The Koreans have picked him up.
But really, he can zone assassins, with his gravity field. Clear waves easily with E. He becomes a pretty good duelist with the Q.
people discover things that are good even though they weren't well known before, especially if they counter whats currently strong. Vik is the replacement for veigar because vik has reliable anti-engage cc just like veigar USED to...
he's been dominating in other regions, and eu/na are just now picking him up. Same for Hecarim
He was a Sleeper.
Because all in is strong now.
And he just has to press R to make all in comps unviable
Don't worry about it. He won't be viable for long if Rito so much as glances at him.
Either that, or he'll be made into a horrid, brokenly powerful abomination. 
Because he does what Fizz did with fewer buttons. Ever gotten a Lichbane with him and pressed Q and then AA'd? And as others mentioned, they do 'all in' comps, so he gets to stun many people and one shot them all while silencing any carries who try to get near him with his R or just Q+AA'ing them as well. With the lack of assassins in 'healthy' positions, people tried to look elsewhere.
People just figured out he can be strong. Same thing for Lissandra. There's no special 'something changed and now he's strong' like there was for J4.
Someone figured out that rushing 
as the first item on him was bad and that his kit had uses in and against certain team comps.
He does a fuckton of damage in 2 abilities and hard counters anything that tries to engage on him
He is a strong answer to engage comps. He likes it when the enemy comes to him, and when they do he can drop his giant AoE CC and then vomit damage onto 5 people.
Somebody in Korea beat an LCK pro with him. Said pro then noticed Viktor actually not being too bad, and proceeded to play him in scrims and training matches. Said pro saw good success with him, particularily in some popular comps, and picked him in actual games.
Said pro might have been Kur0 of GE Tigers (the first Viktor in recent times, iIrc).
