It doesn’t really make sense to me. All the other weather warning names are self explanatory. For example, a hurricane warning forecasts a hurricane. Why don’t they call it a “fire danger” or “fire weather” warning?
The National Weather Service (NWS) has both Red Flag Warnings and Fire Weather Watches.
What are Red Flag Warnings & Fire Weather Watches?
Red Flag Warning A term used by fire-weather forecasters to call attention to limited weather conditions of particular importance that may result in extreme burning conditions. It is issued when it is an on-going event or the fire weather forecaster has a high degree of confidence that Red Flag criteria will occur within 24 hours of issuance. Red Flag criteria occurs whenever a geographical area has been in a dry spell for a week or two, or for a shorter period , if before spring green-up or after fall color, and the National Fire Danger Rating System (NFDRS) is high to extreme and the following forecast weather parameters are forecasted to be met:
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http://w1.weather.gov/glossary/index.php?word=red+flag+warning
Historically it comes from land managers raising a red flag near the ranger station to inform others of dangerous conditions.