Subs are non-directional, as someone else pointed out, and bass sound waves need room to propagate. The actual direction they get "aimed" can also depend on the trunk area and shape.

My subs, while currently pointing backwards, would have been better firing upwards for no other reason than the manufacturer (Audiofrog) doesn't recommend grills. As it is, I have to be careful what I place in my trunk to avoid punching a hole in the cone.