Question

why hot air ballon bursts at high altitude?

Answer 1

Because the atmospheric pressure is too low to contain the expanding volume of the gas within the balloon.

Answer 2

here is your answer:-
as weather balloon travel from the earth's surface, the air pressure around balloon decreases drastically. as the air gets thinner, the balloon's casing get tighter. this is due to gas expanding within balloon. the expanding gas causes the balloon to reach maximum height and it pops.

hope it helped ^_^