It depends on the surroundings and atmosphere. For instance, if you were falling after an accomplishment, like climbing a mountain, then it may mean you are scared that you will fail - even if you succeed.
Or if you start to sink while talking to your mother-in-law, then it could just mean you are tired of listening to your mother-in-law and want to disappear.
If you just fall or sink with no external events taking place, then it may just mean that you are feeling out of control, not normal, or below par.
I think dreams are very personal to the dreamer, and what works as an explanation for one person may not work for another. Take into account everything in the dream (the people, colors, and situation) and, most importantly, how you feel - scared, indifferent, or curious. Then go from there.