Physics! I've heard of it.
Pressure is related to volume and temperature:
- decrease the volume and the pressure goes up
- increase the temperature and the pressue goes up
And vice versa, of course.
So, assuming the tyre doesn't decrease in volume when the tyre is loaded (and the temperature is the same) then the pressure won't change.
In real world use if you put the bike down and load the tyre, the tyre changes shape at the touch point, but the loss of volume where you squashed it down is made up for by the tyre bulging out elsewhere - keeping the overall volume the same.
If you drop the bike down heavily, then there may be a temporary localised pressure increase, whilst the tyre flexes itself into the new position/shape. But that's only very temporary, like a ripple of air inside the tyre.
A real physicist will be along soon to correct me...