Autosuggestion
Intervention by a spirit/ghost
There are a few possibilities that are discussed in these pages. There is at least one more that should also be considered but I don't think it will help much to go too deeply into it. The most likely reason for the glass moving is fraud.
There are most definitely seances in which the glass moves due to fraud. These are usually conducted by professional, but unscrupulous, mediums. On other occasions it is just a prank played by someone, not for money but more for fun or the sake of humiliation.
The manner in which these people pull the fraud off is more James Randi's field.
After excluding fraud, the possibilities we are left with are: autosuggestion, self-delusion, intervention by a spirit/ghost, intervention by some other paranormal creature, telekinesis and telepathy.
