Basically you and the airplane are both in a steady climb and when the aircraft levels off you continue to move up faster so it (via the seatbelt) keeps you at its same level and decelerates your own personal vertical climb. It is just like the feeling you get in a roller coaster.

For the vast majority of a flight, a plane feels nothing like a roller coaster; it feels like sitting in a stationary room with a low humming sound. However, during takeoff and severe turbulence, you may experience sensations similar to a coaster. During takeoff, the rapid acceleration produces a "push-back" into your seat similar to a coaster's launch. In turbulence, the "drops" can feel like the stomach-churning weightlessness of a coaster's crest, but the physics are different. While a roller coaster uses "centrifugal force" and steep drops for thrill, plane movements are usually "shaking" rather than sustained falling. Pilots describe turbulence as "driving over a gravel road" or "someone shaking a box." Most people find the "roller coaster" sensation is purely psychological, triggered by the body’s sensitivity to vertical movement, even if the plane only actually drops a few feet.