Illiterate youtube users often refer to this phenomenon as "trunk bounce", sound bounce and "bounce". Others call it cabin gain, boundary loading, mass loading and transfer function. Calling it cabin gain or transfer function is only partially true, as boundry loading only discribes what goes on with the reflective surfaces in close proximity to the subwoofer istelf. Cabin gain refers to entire vehicle as an acoustic system.
I've put together a visual reference to boundary loading here.
Here we have a positive point source sound wave propogating in free air. Free air = No boundaries. Being a positive wave front, it would represent the cone of speaker moving from rest to forward position.
Next is a point source wave front in free air. This represents forward-positive, to backward-negative cone motion.
Next, shows a speaker in a sealed enclosure, radiating into free space. Notice: The rear wave is isolated inside the sealed enclosure, and the wave in front of the speaker looses intensity with distance. Ever pop your trunk and the bass wasn't that loud anymore?
Next, the boundary in gray, represents the trunk walls. See the wave front impact the walls, building in intensity and reflecting away. This is boundary loading.