They don't really pile up like that. The 'pile up' description of how shock waves form is pretty flawed.
The shock wave forms at an angle off of the body. Generally speaking, it forms a cone off of the nose of the body. Technically, there are usually two cones, one from the nose, the other from the tail caused by the air coming back together again.
As the aircraft flies forward, the cones sweep out a footprint on the ground. When the cones pass over you, you hear the boom.
The aircraft's lift contributes to the strength of the boom. So, the boom will actually be slightly weaker at the end of the mission (because a bunch of fuel has been burned off).
When the aircraft turns, it can create regions of a stronger and weaker shock. First, the lift is increased during a turn. Second, the turn causes the shock to spread out (on the outside) and squeeze together (inside of the turn). This causes some boom focusing and can strengthen the boom.
The atmospheric conditions between the boom causing aircraft and the ground can have an effect -- winds, temperature, humidity, etc. can change how the boom propagates. An aircraft could take off in conditions that dissipate the boom, but at the end of the mission could be somewhere where the boom propagates with fewer loses -- and would therefore be perceived stronger.
When an aircraft slows down at the end of a mission, the strength of the boom changes and this can also cause some focusing like a turn. Similarly, if the aircraft dives or climbs, it can cause focusing or spreading of the boom.
So while the shock waves don't pile up in the sense of accumulating throughout the mission, they can interact in complex ways to affect the strength of the shock from the aircraft.
Engineers working on low boom aircraft not only study the shape of the aircraft, but also the propagation of the boom to the ground and the trajectory the aircraft will follow. It may be that turns are planned to focus the boom in unpopulated areas, or deceleration / descent at the end of missions is done offshore before coming over land... There are many interesting challenges to be considered.