In Hurricane Fluttershy, Rainbow Dash takes charge of the Pegasi of Ponyville to lift water for Cloudsdale. Does she have some sort of official command, or is she just taking charge of her own accord?
According to the wikia, Rainbow Dash is in charge of managing the weather in Ponyville. Presumably this leads into other Ponyville Pegasi naturally looking towards her for leadership in other situations (the events in Hurricane Fluttershy for example).
Beyond that, it's likely the case that she's just highly confident and more likely than usual to find herself leading in various situations.