Usually a fermentation happens quicker than normal for 3 reasons. Poor oxygen, large yeast pitch, warm temperatures.
Low Disolved Oxygen: Yeast needs oxygen in wort to multiply. When oxygen is gone it's one of the triggers for yeast to move from growth phase to feeding phase. This can result in off flavors and mostly poor attenuation, incomplete fermentation
Large Pitch: Beer styles that have clean ester profiles do very well with large pitch rates, even full pitch by using the entire cake or trub from a like batch. Most esters are made during yeast growth, with larger pitches the growth phase is short or skipped all together greatly reducing growth esters. This can be bad for styles that are ester dependant like belgians, saisons etc
Warm Temperatures The warmer the temperature the higher the metabolism of the yeast. Everything happens quicker. In this haste a lot of undesirable chemicals are produced. Bad esters and larger molecule alcohols. Leaving a beer hot from fusal alcohols and even have extremes of nail polish aromas.
Hope this helps.