How does polydimethylsiloxane stick to glass?
If you ask a chemist about PDMS and its adhesion to glass, she will immediately think of the work done in constructing micro-reactors.  This is because one method of manufacturing micro-reactors is to stick a patterned piece of PDMS to a piece of glass; the PDMS is patterned such that it has reactor channels that are sealed when it is stuck to the glass.  The glass is usually a micro-scope slide or something similar.

With the above in mind, in order to answer your question I began to search through the literature which describes the preparation of micro-reactors from glass and PDMS.  What they were saying was that in order to form a bond between the molecules within the glass and the molecules within the polymer (PDMS) the glass had to be pre-treated,


  "most of the research papers mention the
  use of oxygen plasma for developing chemical (siloxane) bonds be-
  tween the participating surfaces"


from "Studies on Surface Wettability of Poly(Dimethyl)Siloxane (PDMS) and Glass Under Oxygen-Plasma Treatment and Correlation With Bond Strength", DOI: 10.1109/JMEMS.2005.844746.

Since the manner in which your material is applied does not have any sophisticated pre-treatment then the only alternative method of adhesion (as pointed out earlier in this thread) is Van der Waals forces.

If this isn't quite the answer you expected, at least it is a good start.