I have read all the other posts here and I'm sure they have pretty much covered the physical aspects of how an object seems to sink down to China. I agree with all of them and can only add this . . .
In addition to all that has already been posted here, a lot of that depth may also come from a time factor. When did the coin get dropped? Maybe it was a 40-60 year old coin in your pocket when you accidentally dropped it one day. For that reason it would of course be near the surface.
There are so many variables in situations like this that a clear answer is hard to give. However . . . all other things being normal, I would think the type of soil, rain, frost, and dirt accumulation are the most likely reasons old coins sink over time.
Hobo