One of the common problems of ceramic tile installed over a wood surface is the weakening of the BOND between tile and plywood caused by shrinking and expansion of the plywood (or any lumber product) substrate which of course causes movement NOT relative to the tiles. All lumber products have micro cells in its composition that hold moisture and when the levels of moisture change ... so do other dimensions of the product as well. Wood shrinks when it dries and swells when it becomes wet. These dimensional changes vary with the species and the orientation of the wood fibers.
The moisture level of the wood will eventually reach equilibrium with that of the surrounding air This equilibrium moisture level depends principally on the relative humidity of the air and of course the humidity levels change substantially from season to season. Many believe that keeping the temperature constant inside a home or other building will decrease humidity changes .... this isn't so.
Metal fasteners such as staples, nails and screws are also effected by the dimensional changes of the wood and will sometimes lose some of their grab and even be "pushed" part way out of their lodging. Screws are the least effected of all fasteners.
This website uses a forum to discover better ways and procedures for installing ceramic tile from pros all over the world and most would agree that tile installed directly over wood isn't the best procedure regardless of adhesive used. There needs to be a membrane between the wood surface and the set tiles which allows for independent movement of the substrate thus having no effect on the tile installation.
Some of the wood products used as a subfloor are:
Oriented Strand Board (OSB)
Not only do these types of subflooring contribute to various forms of movement, but also floor joists are subject to dimensional changes as well. All these factors should confirm that other requirements are needed for a successful tile installation.