Why is mundo jg suddenly meta?

In patch 9.16 they reduced the cooldown of his E from 6 at all ranks to scale down to 4 at rank 5. You max E on Mundo in the jungle and this change increased his power by a fair amount when maxing E. In lane you typically want to Q max for poke damage which pushes Mundo into the jungle to make use of the buff to his E earlier in the game.

If you are unaware of what his E does, at max rank it gives him 42% more magic resist, 100-200 AD based on his missing health, and deals 5% of his max hp as damage in an auto reset. To put this into perspective, that is up to 7k gold worth of AD and anywhere from 300-2600 gold worth of magic resist for free. That isn't even to mention that it does more % health damage than grasp on a lower cooldown (with cdr) and is an auto reset.

I'm not completely sure, but I think it has to do with grievous wounds. When you jungle, most of your fights will be against the enemy jungler. And junglers don't usually take ignite or rush executioner's calling. Also cinderhulk is a really good item for Mundo, especially when combined with spirit visage and warmogs.

He's been buffed many times in the past few patches, making him a bit overtuned. He already had the best clear in the game, and Riot made it even better with all the buffs they've been giving him. He's not kited as easily anymore, especially with Fleet giving him bonus movespeed and having tenacity built into his kit. He's just an overall good pick, and putting him in the jungle eliminates laning, which he usually struggles with against a lot of matchups due to subpar early.

Because goes where he pleases.

i still dont think he has great ganks, he really need laners to set it up with cc considering other then his slow on Q he has none. now taking blue smite helps but he does scale well in general. and things like cinderhulk giving +15% bonus hp scales well with him over all

Because his clear his insane with his kit+cinderhulk, while he scales massively with cinderhulk at the same time as he inevitably builds tankier too. His ganks are very simple, while he doesnt have hard CC like many junglers, he can still catch out one champion with the help of the ally laner especially if they are pushed up. They also buffed his E which is why people max it first in the Jungle, both letting him clear even faster while letting him chunk an enemy when he gets close enough in a gank.

tiamat mundo goes where he pleases.

Had a Mundo jungler earlier who said "three losing lanes great" at 15 minutes after ganking absolutely nobody. He continued to be negative and not participate in a single fight until the nexus turrets were being knocked on, at which point he finally picked up a couple of assists.