The possibility of your phone catching on fire is almost slim to none. However in incidents that have arose, it is usually because of the battery. Samsung had a Chinese user of the Galaxy Note 7 experienced a phone catching fire and trying to ensure this doesn’t happen to anyone else.
Samsung said in a statement on its China website its investigation of the phone shows "the damage to this product was caused by external heating", without elaborating. Moving forward this could impact the use of Samsung Phones because of battery concerns.