I can't speak to the timing, time, or location of these events, but as for the difference, sequence, and cause of them...
It's been my understanding that there were a number of smaller 'oscillations' culminating in a more significant 'pressure bump' at the end. Ostensibly the oscillations would have been caused by Cooper moving down and up the stairs, which would be slower and less pronounced, ending with the bump when Cooper exited, which would be the most sudden and pronounced effect of his weight on the stairs.
That makes aerodynamic sense to me.