Ummm… Grease and heat is a good start. Clean oil with a higher flash point is also a good start (if your oils starts smoking, your really pushing it beyond its limits), along with cleanliness.

A greasy hood frequently is the cause, ya gotta get them cleaned regularly.

Biggest screw up would be using an old deep fryer that doesnt have a high temp cut off. If your thermostat goes out your oil temp will keep rising until fire. Keep a class K Fire Extinguisher on hand for a grease fire