This is a very good question which I have asked myself, and have others
ask me, for years.  This is generally what I say.

Framing pushes the artifact against the glazing with some force and
holds it there more or less permanently.  The close contact creates
capillaries which, if water should condense into, can only escape by
evaporating one molecule at a time (slowly, thus long contact time).  In
addition, experience has shown that contact tends to encourage bonding
between the media and the glazing.  This could have several causes;
constant temperature and humidity changes undoubtedly play a part.
Materials which are wrinkled or have wrinkles tend to be polished on the
surface in contact with the glazing.  This also probably has something
to do with constant swings in temperature and humidity and thus
expansion and contraction.

Do not forget that some display lighting heats up and dries out the
interior of a frame when the heat is radiating from the bulb onto the
glass.  Dark surfaces tend to hold more heat.  When the lights go out at
the end of the day, the temperature drops, cellulose expands and RH
increases.  In addition, on a run of days with high humidity, the
interior of the frame can pick up moisture during the daily internal
heating of the frame.  When the conditions are right, cold air on the
glass can result in large or small amounts of water condensing on the
cool interior surface of the glass.  This moisture and capillaries have
to play some role in the adhering problems.

The polyester encapsulation film is generally not held tightly against
the object.  In addition, the polyester is permeable to water.  It does
not move quickly through the plastic, but it does move from areas of
high concentration, to low, by diffusion through the plastic and out of
the enclosure.

I have only seen one piece of polyester stuck to the surface of a color
lithograph.  It was pealed away the way tape is remove at a very tight
angle.  I have seen several objects stuck to glass.  High humidity or
water was generally involved.  I have seen the surface of several matte
objects polished against glass.  I have even seen small areas of a
thickly applied media bond to the inner surface of an acrylic sheet (no
hope for that one).

Tim Vitale
Preservation Associates
Aptos CA
831-684-2731