Working environment - Altitude

In order to comply with some safety regulations and standards, especially related to creepage and clearance, the insulation characteristic of the air is considered. At high altitudes, the air's insulating characteristic changes, and so does the the electronics' behaviour. In the case of oscillocopes, it is probably even more sensitive due to its high speed electronics / bandwidth and the possibility of reducing the isolation between some traces or components (source of noise due to coupling).

More to that in here

Your oscilloscope probably depends on proper convection to eliminate heat from the electronics. As the air gets thinner it is less able to remove heat from the components, so there might be a possibility of overheating at high altitudes.

The spec sheet gives a transient voltage rating of 1000V. With a tight circuit layout, necessary for fast response, that may arc at high altitude.