Autosuggestion
Intervention by a spirit/ghost
This is where a spirit or ghost of a dead person somehow controls the movement of the glass.
If this is the mechanism a seance should be able to answer questions that a participant knows the answer to, questions that could be answered by something not constrained by physicality, questions that could only be answered by someone who lived in the period that the spirit comes from (but could be confirmed by good historical records) and questions that could only be answered by a creature with the abilities that are claimed (ie if the creature does not claim to be able to predict the future, not being able to do so does not disprove the seance's validity).
Testing such seance is covered in depth in the seance test page. Basically the test should hinge on the spirit or ghost not being constrained by walls or other physical objects in the same way as people or being able to answer a question that can be confirmed as true somehow but could not be answered by the participants.
Note that a number of people who have written have pondered the motives of the spirits or ghosts. One suggested that the spirits that are around tend to be either preoccupied (in which case you won't get to talk to them) or hanging around because they cannot get to the next stage, or whatever. The upshot was that these types are likely to be bored and/or bitter and therefore would think it fun to play tricks on seance participants. Attempts to confuse these poor patsies, lying to them, pretending to be demons or angels and other larks would be likely.
The point is that even if the spirit claims to be your long lost aunt, there is no guarantee that who you are talking to is who it says it is (see also Demons, etc).
But how would a Spirit or Ghost move the Glass?
How indeed. I'm thinking about this and will put more here when my thoughts crystalise into something I can get down in writing. Some very basic thoughts are included below.
It is a possibility that the spirit puts the answer into the minds of one or all of the participants who then control the movement of the glass. If this is the case you should be able to run a seance in your head. ("Tell me when I get to the right letter!") Based on an assumption that a spirit is involved, this is the most likely mechanism to me.
Alternatively, the spirit may use the telekinetic (or psychokinetic) ability of one or more of the participants to move the glass, although this quite unlikely to my mind.
Then again, there maybe some sort of poltegeist power involved. I find this a difficult option to consider reasonably because you are assuming some sort of direct transfer of energy to move the glass. Getting your mind around the possibility of a spirit providing an answer to a question is hard enough without throwing in a variant of free energy theory as well!