its mainly because everyone else got nerfs, so he made a resurgence. 8 months ago no one but diamondprox even played lee in competitive play lol. Notice how amumu got no buffs and he went from unplayed to "OP" at lower elos. Same thing with malphite he went from unplayed to also "OP"

He has been always good. There could be multiple reasoning, like armor pen formula changes, more build path options, wriggle users got better, ambient gold increase (made characters who required more gold to work, WORK) etc.

Although, I believe these nerfs are targeted towards his laning which has been broken for a long time.

After the jungle changes, Lee Sin's clear speed and ganking potential remained unchanged, then he became the best jungler in the game, keeping his level 2 gank, being resourceless, outclassing many other junglers who's jungling became harder.

What do you mean "suddenly"? Lee has been an extremely dominant pick basically forever.

Suddenly?

he's getting a nerf so w/e
they have #s that show he does too much dmg and nerfing him will allow them to hold onto the cleaver and what have you. if the items were nerfed instead it'd just destroy some champions