As it moves faster over the top than it does on the bottom it creates less downward air pressure. You may be asking why the air needs to reach the back of the airfoil at the same time. Air is a fluid and occupies space - if you push a basketball down into a tub of water, the water level throughout the tub rises because the ball is taking up space. An airfoil is creating a similar effect in the air. The preceding laws plus gyroscopic stability and gyroscopic precession complete the science of a returning boomerang.
Gyroscopic stability is something everyone has experienced.
Try balancing on a bicycle without rolling forward and you understand the stability of spinning wheels. We have all spun a gyroscope or top, the simple act of spinning it creates stability. That gyroscopic motion keeps rockets on course and bicycles upright. It is the same spinning motion that gives the boomerang stability in the air. This increased lift at the top of the circle begins turning the boomerang and it begins its return.