Real answer, Game Freak. Interesting answer, it has to do with the behavior of the Pokémon.
Thanks to a behavior of theirs known as “synchronizing,” an entire flock of these Pokémon can attack simultaneously in perfect harmony.
-Violet Pokédex entry.
Flamigos seem to be so skilled at fighting that an entire flock can attack together with perfect harmony. The Flamigo is a Fighting type due to its skillful fighting behavior. Hope this helped!