Why does dragging a clip change its sound?

Ducking is usually taken to mean that an audio signal power is reduced when another signal is added or increased. If you are saying that the moved signal sounds less loud when moved to a position in which other sounds are present, you may just be experiencing a psychoacoustic phenomenon known as masking. The auditory apparatus and the brain perceive a given sound as less loud when other sounds are present. Put a meter on the solo track and see if there is really a drop in amplitude. If you are saying that the clip is less loud without other audio being present, then it is odd to describe that phenomenon as "ducking."
 
Masking does not just affect perceived loudness, but can affect pitch and timber perception as well. In addition, the physical phenomenon of destructive interference can reduce the actual sound pressure level when other sound is present, and by affecting different frequencies which in turn are perceived as different loudness along the Fletcher-Munson curves, can affect loudness, pitch and timbre. 
 
Short answer, changes in the sound of a clip are to be expected if the clip is moved relative to other audio. How much change should be expected is too complex to predict.