One of the most common reasons I find is that for one day out of every week, moderation and balance are foreign concepts and one may tend to fly off the charts on their caloric intake. This not only can negate your entire calorie deficit you've created for yourself but it is also commonly an indication that one's relationship with food is still a dodgy area. Any time you're feeling like there's a lack of balance, it is time to take a step back and figure out how to better get our heads on straight:)
I call this graphic "Why an entire cheat day is a bad idea". I hate the word cheat, but since it's commonly used to describe what is happening here, we'll use it. All the calorie information from Saturday was taken from actual chain restaurants.