Pressurisation of cabins usually results in fourfold higher pressure inside an aircraft compared to the outside environment. High-pressure air will always move towards low-pressure air, which leads to a force on the airframe trying to expand the cabin, putting the skin under tensile load.

This can be described as a balloon being inflated more and more. Due to the pressure from inside, the skin stretches (tension) and the circumference increases (obviously the circumference of the fuselage only increases fractional – compared to the balloon)