Déjà vu is a French term that literally means "already seen" and is reported to occur in 60-70% of people, most commonly between the ages of 15 and 25. The fact that déjà vu occurs so randomly and rapidly—and in individuals without a medical condition—makes it difficult to study, and why and how the phenomenon occurs is up to much speculation.
A related theory states that déjà vu is a fleeting malfunctioning between the long- and short-term circuits in the brain. Researchers postulate that the information we take in from our surroundings may "leak out" and incorrectly shortcut its way from short- to long-term memory, bypassing typical storage transfer mechanisms. When a new moment is experienced—which is currently in our short-term memory—it feels as though we're drawing upon some memory from our distant past.
What triggers it?
The actual trigger for it in healthy individuals is not exactly known, but we do know those same regions of memory and memory monitoring are involved.
One explanation for déjà vu is that there is a split-second delay in transferring information from one side of the brain to the other. One side of the brain would then get the information twice – once directly, and once from the 'in charge' side. So the person would sense that the event had happened before.