Tank Builds on assassins?

So recently, to my knowledge Fiora got a new favourite build which is the hydra bruiser/tank build, I don't really know what sparked the change but it happened (If anyone could give insight on the change please do). Now about a week or two earlier Akali got reintroduced in korea, not much as an ap assassin anymore but as another hydra tank. What's going on? Has it been sort of decided that the assassins that don't have escapes would be better off doing damage this way, surviving teamfights and doing damage through titanic hydra and either true damage or high base damages? I really would like to know what's behind this, to be honest though I dig the KR akali build so far.