This is my theory on The Bite of 87, who caused it, and why Foxy is out of order in the first game. Enjoy!
In the second game, there are two animatronics that could likely be the ones who caused The Bite of 87, Foxy and Mangle. Foxy isn't out with all the other new animatronics in the day, but he doesn't have the face recognize things that the new animatronics do. Foxy could have not recognized the right face, and bite the wrong person thinking it was his target. It is likely that no kids found him, and Foxy bit their frontal lobe off, so it must have been someone not hidden away, like Mangle. Mangle could be mad that the children treat him like dirt. He kept getting torn up and it happened so frequently, that the employees got tired of putting him back together and he was turned into a "take apart, put back together" attraction.
Now that we know the most likely animatronics, lets figure out what age group the victim was in (child, teen, adult). Now, Jeremy is the most likely, because he had the day shift on the said day of the bite. He is told to stay close to the animatronics, which would mean he would be easier to bite. Jeremy wouldn't let his guard down, but then it would be easy to get close to him and bite a chunk of his head off. Since the new animatronics have the face recognition, they would not attack a child. The old animatronics, however, don't have the face recognition, but they still would not hurt children. Mangle wouldn't attack a child, more likely an adult. Foxy wouldn't attack a child, either. The animatronics were reported to be different around adults, which means they would bite an adult without blinking an eye.
The only way Foxy could cause the bite is if he was after somebody, because no one would attack someone for no reason, and know the consequents that will come. The only reason Foxy would be mad is if he is mad about the old place closing down. Maybe Foxy tried to kill the man/woman that killed the children, and Foxy tried to kill that person thinking that they were always in the building. It doesn't state in the game if the person that murdered the children was a employee, meaning Foxy could have confused somebody for another person. It is likely he did not, because Phone Guy might have been the killer. Now, I'm not going to get into to much detail, but, Purple Man in the mini games has a phone. That could mean that Phone Guy could be the Purple Man, which is the murderer of the five children, plus more. In the mini games, Purple Man kills the crying child in the Give Them Cake mini game, he also kills the children in the Foxy mini game (sorry, I don't know the name of it :/).
Mangle is out in the open, waiting for its victims to come along. Mangle could easily get close to an adult and bite them without the adult knowing. Jeremy had to stay extra close, and it would be easier to get a bite out of his head. Since the second game was a prequel, the animatronics had to be scrapped, so they could not be in the first game. If an animatronic were to malfunction, Fazbear Entertainment would have to scrap them, and possibly scrap a whole set of animatronics. If Foxy bit someones frontal lobe off, he, and possibly his friends, would be scrapped for good. Mangle, or some of the new animatronics, bit somebody's frontal lobe off, not Foxy, not Freddy, not Chica, and obviously not Bonnie. Mangle is likely to do the bite for one big reason, his anger from being ripped apart and shoved back together, had taken control of him and bit someone.
Since Foxy couldn't have caused the bite, then why is he out of order and why is his jaw unhinged? Foxy might have been put out of order because kids were ripping him apart. Fazbear Entertainment was about to lose all their money, and they would not be able to open another place. They spent most of their budget on the place from game two, so they had to have a smaller place to still have money left (after it closed down). They lost money because an animatronic was mad at being taken apart. They had to put Foxy out of order before they get closed down for good. Foxy was torn apart and his jaw was unhinged by the children, not by biting a frontal lobe off.
This is my theory of The Bite of 87, and I hope Everyone enjoyed. Remember, this is just a theory, and I began writing this after an idea popped into my head. This was not thought out a lot, only enough to pass as a FF.
