If you look at the exact relation of the piggy-back shuttle to the carrier 747, the shuttle is tilted nose-up by several degrees (also true for the Shuttle at launch, a slight angle exists between External tank and Shuttle)

The reason for this is that without an angle, the Bernoulli effect of the fast airflow in the narrow gap between the two craft would suck them together and cause damaging loads crushing the supports.

With an angle, the ram effect of air entering a wedging gap gives back-pressure to balance the suction.

There's far more, of course, like how pieces of foam get sucked off one piece and twirled around before hitting another...