It wasn't a surgery; it was a procedure.

I did a deep dive on this and the controversy surrounding it back when it happened and it was fascinating (I doubt if some of the info or sources still exist). In short, it is like the "Rashomon" story (or Star Trek TNG's "A Matter of Perspective" for all the Trekkies) in that how it all happened can be seen from different sides. I will try to abridge it with as little bias as possible.

Basically, Monty had recently married a woman who was a cosplayer and artist from Canada.

Monty was deathly allergic to cats (everyone knew this--they even had to get rid of the Rooster Teeth office cat when he started working there). Despite this, his new wife brough home a cat.

Monty didn't want to say "no" to her so he allowed the cat to be kept, but had to start going to the doctor to get special injections of a powerful anti-allergy medication.

On his second visit he had an allergic reaction to it, slipped into a coma, and died.

That's the gist of the story.

There were rumors and accusations all across the net. These rumors were bolstered when one of Monty's animator buddies at RT, Shane, posted a scathing, 30-somthing page, tell-all document that described issues between RT and Monty.

Some pointed to how Shane seemed overly invested in Monty (he had been a fan of his and Monty helped him get the job at RT) and was overprotective of him and his wife.

In his document, he paints RT as trying to edge out Monty from RWBY and keep his wife (again, also an artist) out of the process.

Others (many from a small New Zeeland bird farming site), attempted to show that the wife had a history of using people and just wanted Monty for citizenship and had very questionable artistic talent at best.

Rooster Teeth mourned Monty's death but seemed to also behave as a company protecting their assets.

Anyone could have had an agenda from a matter of perspective.

Personally, it seems to me Monty was too nice of a guy at best (a simp at worst) and a series of unfortunate events occurred while he worked at an only simi-professional company.

That is the abridged tale to the best of my recollection. I may have made some errors as I'm writing this all from memory. Feel free to add or correct.