Nothing is wrong. The TrekGuide will automatically switch from the barometer mode to the altimeter mode in two circumstances: when moved from a resting position, or when moving and in the barometer mode.
The former happens because the unit is designed to show the most useful information and usually when you are moving and climbing you will want to see the altitude. The latter occurs because the TrekGuide’s altimeter mode uses less current and it goes into this power saving mode to save the batteries. (The estimated battery life is more than one year.)