There is a direct correlation between the two, higher altitude and lower boiling point.
Two factors combined, i.e. early boiling & drain mechanism of pressurized coolant, result in the problem.
Higher altitude means thinner / less condense air, this leads to less efficient cooling mechanism or less effective radiator thus retaining more heat in the system. Secondly, water boils at much lesser temperature there, sending vapors and water out to the reservoir from where (vaporized water due to lower pressure) tends to escape thru the overflow pipe or any leaking joints.
Different radiator caps are recommended for very high altitude driving. Don't remember the specs right now but there is quite a bit of information available on net about this exact subject.