As the aircraft rotates (lifts off), the springs on the landing gear naturally extend to their greatest extent. Given the strength of the springs, there is a bit of bang as they reach the end of their travel. The same would happen if your car suddenly left the road on a hump-back bridge!
Only diff - in your car you would not have time to notice the bang as you worried about the heck was going to happen next!!! Sitting in your seat as (something like) 350 tons is lifted off the springs, you notice the 'bang' but it is a nice bang!