At my previous airline, the procedure for Cabin Crew to initiate an evacuation initially allowed for 3 situations in which they could initiate it: 1) Ditching, 2) Aircraft broken up, or 3) Major self sustaining fire. They then chose to remove "Major Self Sustaining Fire" from the list of situations in which cabin crew could initiate an evac, as cabin crew would not be able to tell if a fire was self sustaining or not - and the flight crew would be aware of any fire that was self sustaining, and be able to call for the evac themselves.
I fought this decision strongly, making particular reference to British Airtours flight KTM28 (737-200 which had an engine failure and subsequent fire during the take off roll. The intitial engine failure made a sound like a large "bang" which the flight crew presumed was a tyre bursting, and initiated rejected take off procedures. Unfortunately though, the ensuing fire from the engine failure was catastrophic, and was unnoticed by the flight crew - who used reverse thrust to stop the aircraft - therefore fanning the flames, and to make matters worse the prevailing wind then fanned the flames directly onto the aircraft. My point was the Flight Crew are not neccesarily always going to be aware of a major self sustaining fire - more detail on that incident
here)
In any case, logic should/does prevail - and I would presume in the Alaskan case it did, we just dont have all the facts yet. As a previous poster mentioned, the engines would most likely have still been running, hence the delay in the call for evacuation - but of course the punters dont know/care about these kinds of things...