Because it happens from pressing a button and the screen goes dim is that maybe they are pulsing the display to vary the contrast. The pulsing can occur from a loop or from an interrupt. The keyboard can be tied into an interrupt with higher priority which could cause more cpu time to be consumed upon pressing a button, thereby limiting the pulses to the display. I would imagine that they would pulse the display because as an LCD is very environment dependent upon temperature. They probably have a temperature feedback sensor to adjust the contrast by pulsing the LCD depending on what the temperature feedback reads. Just for verification, I know that my TI-85 calculator would get a dark screen when it was hot and I would have to turn the contrast down and I know when that it was cold, I would have to to turn the contrast up. That could be reversed, it has been a long time since I used my calculator outdoors. A better approach instead of pulsing would be to use a digital pot and vary the contrast based on the temperature reads, but I do not know of a digital pot that would work with negative voltages or at least I have not needed to use one (I guess you could use relays and put resistors in series and short out the ones not in use making a binary resistor). One could also use a separate CPU for the contrast control if there is this CPU timing problem thereby, avoiding the power needed to drive the relays. A video showing PWM to vary contrast:

http://www.youtube.com/watch?v=qT7tq4rN8LE