descend to, lapse into, degenerate
墮 centers on a downward movement into a worse state: descending physically, lapsing morally, or degenerating in quality.
墮 is the traditional form of 堕. It combines 隋 (which likely contributes the sound) with 土 (earth), suggesting falling to the ground. The exact historical development of 隋 is uncertain.
The top 隋 looks like a structure collapsing, and the bottom 土 is the earth. Picture something crumbling and descending to the ground, degenerating as it falls.
For ダ, imagine a heavy object falling with a dull 'da' sound as it hits the earth: da -> ダ, and the impact marks its descent.