Why do flames shoot up from restaurant pans?
There's no optical illusion, what you are seeing is real flame shooting up. In a kitchen flare-ups have 2 common sources - alcohol and fat. Alcohol in concentrations 50% and above is flammable when cold, you add some brandy, vodka, etc and away you go. When heated 40% alcohol will burn in the pan, anything above 40% can be used for flambes. 

Fats (ie lipids as in oil or fat from animal of vegetable sources) can also burn when very hot and exposed to an open flame as anyone who barbecues will know all too well. Fat flares can happen in any kitchen in the right conditions (even at home), say when something is being vigorously stirred and a bit of hot fat splashes down the side of a pan and ignites, trailing back to the pan itself and giving a quick burst of flame. 

As for how often it happens it really depends on what is being cooked, the equipment and method used. If there's a lot of steak action happening then some flames are pretty normal, if they're making vegetable stew ... not so much. It could be a bunch of chefs being showy for the camera, or just going about their normal business, it's very hard to say.