Because there's some type of artificial gravity at work on the Star Destroyer. When your pod ejects it's still within the envelope of the gravity.
-Rob
Oh, the question was about gravitational mechanics and not about the ride mechanics.
The simplest explanation of feeling a 'drop' in space being ejected from a starship is that you're being ejected from a starship. Acceleration in the direction of one's feet offsets any sort of artificial gravity the pods or starship has.
If one was in an artificial gravity field of a starship and you jumped off a catwalk, you'd feel like you're dropping. And if you were yanked down off that catwalk... same thing.