So, I hope guys know, Fnaf 2 is a prequel.
So, my theory is, that before the Fnaf 2 game started, there was a guard before Jeremy and Mike. And when that guard was there, Golden Freddy had an endo-skeleton and worked as an animatronic should do. Then that guard was either fired or left. Then Mike came along. And he saw the chance to kill children in some way. One day, when the animatronics were shut down for their servo's, Mike took one. And he took Golden Freddy, dismantled him, hollowed out his animatronic parts, and left the suit. When Phone Guy tells you he left for "reasons", Mike can see his time to strike. When you, Jeremy, on the 5th or 6th night, was told that Phone Guy was going to take your shift for you to attend a party to check over the animatronics and the children, is when Mike snuck to the place where the animatronics are kept, is when he used the hollowed out animatronic suit to lure the children back stage to brutally murder them and shove them into the suits. The reason you see Golden Freddy hollowed out as soon as you start, is probably because Mike hollowed him out when he left for his day job, and the mechanics never gave Golden Freddy his endo-skeleton back, thus he stayed hollow.
So, there is my theory. You can feel free to use my theory on other websites, etc, but I would like if you could give me a link or some credit? But yeah, this came to me only a few minutes ago. Hope you enjoy :>