Well, it's not excess air as such - just part of the cycle, and perfectly normal.
Air supplied by the solenoid fills the space between the bolt and the can, holding the bolt in the rearward position. This prevent the air in the firing chamber pushing the bolt forward and firing the marker.
When you pull the trigger, the solenoid switches, the air in the can is vented, which allows the air in the chamber to then push the bolt forwards, and the marker fires.
What you are feeling is the air from the can being vented through the solenoid when it switches.
Fact is that any pneumatic solenoid will vent some air as it cycles - it's the nature of the beast. It just happens that the position of the solenoid - and the position of the vent holes on the solenoid - happens to make the venting much more noticeable on the Geo than with most other markers.