Temporal Lobe Seizure - A popular scientific explanation of the phenomenon is that it is a symptom of a coming seizure. Doctors have noted that people who suffer from temporal lobe epilepsy experience vivid cases of deja vu before an attack occurs. Researchers believe that the sensation is an effect of your temporal lobe’s nerve connections doing a “misfire” of neurons.
Hippocampus Abnormality - Another explanation is linked with the hippocampus area of the brain. The hippocampus is where memories are produced. Inside the hippocampus is a small part called Dentate Gyrus. Dentate Gyrus is the one responsible for memories that are episodic – small fragments of memories that form one big memory. This part of the brain allows you to tell similar places and situations apart.
Recognition Memory Theory - One other explanation is the Recognition Memory theory. According to this theory, there are two kinds of recognition memories: recollection and familiarity.
Recollection occurs when you see a man in a supermarket and recognize him as your neighbor. Familiarity occurs when you see this man but are not able to pinpoint exactly where you know him from. According to this theory, deja vu is a form of familiarity.