i've not noticed this--can you elaborate? my first thought would be that the gecko's bones are weak and flimsy due to metabolic bone disease but i'm not sure that could be the case if more than one person is seeing this in multiple geckos...
it doesnt shake when there's weight on it only when its in the air almost like its looking for the perfect place to put it down. when i pick them up they put a lot of pressure against my hand with their legs so i know that they have strong bones.