formatting link
The Reservoir Fire Department in Mississippi made this super dramatic video showing just how dangerous it is to put a frozen bird into a fryer. The reason is relatively simple science: The water from the frozen turkey will sink to the bottom of the fryer (because it?s denser than water) which then in turn will try to escape as steam. When this happens it displaces the oil, causing it to bubble over and when that reacts with a flame you have a giant turkey fire.
More than 1,000 fires are caused each year by turkey fryers. Heck, FEMA (yes, FEMA) released a brochure on the subject.