I can't speak to why fire drops are done in a particular way.
As for the MD80s, if flaps are beyond 26 degrees without gear down/locked, you get a non-silenceable warning horn. I can't tell the configuration from that picture but a normal landing flap setting is 28 degrees so maybe the fire drop was trying to get a particularly slow IAS by using more than 26 degrees so had to lower the gear to prevent the horn ?