Answer
Charged objects, such as the foil leaves, lose their charge more quickly at higher altitudes.
Work Step by Step
The foil leaves collapse in time as excess charge leaks off them. Air is an insulator, but not a perfect one.
At higher altitudes, cosmic rays (high-energy charged particles from space) ionize some atoms in air, and these charged ions provide a conducting path for the foil leaves to dump their excess charge.
The ionization effect is less apparent at lower altitudes because there are fewer cosmic rays there: they are absorbed by the atmosphere, and also decay radioactively.
