The information from after action reports involving firefighters being killed at these incidents indicate that little or nothing was showing at the start of the time line but suddenly without warning the roof or floor failed. This is very difficult to understand if we were laymen. The facts might be better understood if we factored in that during the response all firefighters were being charged with adrenaline.
Upon hearing the report that nothing or little was showing this adrenaline surge begins to wane and a sense of routine can take over thus suppressing the natural sense for caution. This behavior change must be trained, drilled, and instilled - no incident is ever routine. We must accept that we are not always required to rush into a situation. Sometimes it is better to go slow and observe everything then make decisions. Stay Safe.