Crested geckos can drop their tails for the most obscure/random reasons. It really just depends on your gecko.
My male F'lar had a really beautiful tail once...then one night for no apparent reason (or at least that was what I concluded

) he dropped it. F'lar was...is a pretty flighty gecko. So I'd assume almost anything can startle him.
Prevention...
-Just try to keep the gecko in low traffic areas
-keep other pets such as cats/dogs out of the room
-try to keep sudden bursts of sound to a minimum
-when picking up the gecko /approaching cage/waking your gecko to pick up... do your best not to make sudden movements