It's dialectic -- there's lot's of them in the US, but this one afk belongs to Black American English, and is shortened from "fixin' to." Personally, I think it's cool to see so many variations of English. The language is definitely not static; it is changing all the time!
It shows up in US Southeastern slang quite a bit. Usually it's a bit stronger than 'going to,' ex. "I'm fixing to give him a piece of my mind." 'Fixings' is also slang for side dishes here.
As a Southern gal myself, I got whiplash from the implication that people sayin "fixin" are trying to sound important.
I promise, among those of us who say that kinda thing, it's seen as a mark of ignorance and bein low class in general. The idea that a hillbilly accent can seem "important" is banana sandwiches to me.
I wouldn't say it's trying to sound important as much as it is trying to sound serious / no bullshit tone: "I'm fixin' to whop yo ass", or as a response to your boss bitching at you to do something: "yeah yeah I'm fixin' to"