What might make sense for 'oscillations'...
When Cooper opens the door, they drop somewhat but are being held up (partially closed) by the air pressure of the relative wind. When he ventures out on them to jump, his weight causes them to lower somewhat, which would affect the pitch (tail up/nose down). When he jumps, the stairs recoil back up, past their 'static' position. (In fact, they apparently almost closed, which caused the 'pressure bump'.) Then they would fall back down again, again pushing on the air. They would bounce up and down on the air a number of times before settling back to the 'static' position. each time giving a bit of pitch influence.