What is happening with the "Totally Not a Cow Level" buff?

Solution 1:

Update: With patch 2.3 it is now posible to open a portal to this area by placing a Bovine Bardiche in the Kanai's Cube.

There is a chance of running in to a special goblin like Cow called "Herald of the Queen".

If you have sound on you will hear mooing when you get close and a cow icon will show up on your mini map.

Killing this will open a portal to a place that dose not exist (zone name is "Not the cow level" when you get inside):

Inside you will find The Cow Queen:

And Ghost of the Cow King that will start an event:

The even rewards xp, gold and 4 cursed chests:

Other than this the area is full of cow mobs, dead farmers that drops some gold, a lot of chests and a lot of shrines.

Some of the health globes spawn as T-Bones:

The full map looked like his once I was done:

Buff did not change while inside the level.

I have run in to one two time, once in act 1 and once in act 4, both times in zones related to a bounty.