The idea is, that as you go up in altitude, the air pressure outside lowers. This then causes the compressed air in your tyres and forks to expand. The idea is that it may expand so much that it causes the tyres to pop or the seals in your shocks to blow.

However as already stated the storage cabbin is pressurized like that of the passanger cabin. However lots of airline companies still make you deflate your tyres due to the fact that if an air pressure problem in the plane (however something happening like this is extremely rare) then the compressed air may expand and explode.