From Mr Boeing's Maintenance Manual. Chap 12-15-03
WARNING:
A TIRE MUST INITIALLY BE INFLATED WITH NITROGEN. IF TIRE WERE INFLATED WITH AIR, VOLATILE GASES GENERATED BY AN OVERHEATED TIRE COULD COMBINE EXPLOSIVELY WITH OXYGEN IN THE AIR INSIDE THE TIRE. A TIRE EXPLOSION CAN CAUSE DAMAGE TO EQUIPMENT AND/OR INJURIES TO PERSONS.