Five Nights at Freddy's, or FNAF, for short, was born. Players reacted negatively to his previous game, Chipper & Sons Lumber Co., where the main character Tyke (a beaver's son) resembled "a scary animatronic animal," and reviewer Jim Sterling deemed the game "unintentionally terrifying."
Five Nights at Freddy's is a family-friendly pizza restaurant named after its mascot, Freddy Fazbear. Players must survive each night with limited power, while keeping the sentient and hostile animatronics at bay. The most shocking incident happens in 1987.
The Bite of '87 was an unfortunate event that took place in 1987 at the second Freddy Fazbear's pizza, which was briefly mentioned by Phone Guy, a former Freddy Fazbear's employee who assists the protagonists with alarming information and warnings. The newsworthy incident was linked to the company's reputation declining, leading to a major loss of business at the restaurant's many locations.
Management made it so that animatronics would not be permitted to wander around the building during the day, and their free-roaming mode would be limited to nighttime. This is perhaps the main reason for the game's overall premise.
The Bite of '83 would be a similar situation where younger children are bullied into "giving Freddy Fazbear a kiss," which results in him being pushed into Freddy's gaping jaw, only for the animatronic to press down on his head, destroying it.
Foxy's teeth have long been speculated as the animatronic responsible for the Bite of '87. There's a sign that reads 'Sorry! Out of Order,' implying that Foxy fell into disrepair following the incident and was put out of commission. The actor's jaw also hangs loosely, suggesting that considerable force might have disjointed it in the second game.
Freddy Fazbear himself is thought to be the culprit; one of the 'Rules' boards located in the East Hall in the original game states, "Don't touch Freddy." This rule may have been implemented after the Bite of '87, according to one analysis of Freddy's face. This is also proved in Five Nights at Freddy's 2, when Freddy is seen wearing a different, print-less mask.
Mangle's jumpscare animation shows him swinging down from above, opening wide, and biting the player in the head, just where the frontal lobe would be located. There's also a reason for Mangle to be vengeful against youngsters specifically.
The Bite of '87 has dozens more theories, but no one knows the truth. In fact, it has never been verified whether the victim is a child or not, so it may very well be an adult.