If you've ever taken a beginning psych course, you've learned about Pavlov's dogs. Ivan Pavlov was a Russian physicist who rang a bell whevever he fed the dogs in his laboratory. Before long, the dogs had so strongly associated the bell-ringing with the upcoming lunch that they salivated whenever they heard the bell. In other words, the sound of the bell by itself triggered them to expect and to want food.
Humans have triggers, too, situations or feelings that we have come to associate with food. For instance, sitting down in front of the television set is triggering for me, because in my pre-surgery days that was my favorite place to binge eat.
Being stressed out about something is also a major food trigger, since I used to cope with stress by stuffing sweets in my mouth.