Why does BusyCal on iOS create all day events at times and other times not?
By default, BusyCal for iOS creates new events using natural language.
So, typing Staff Meeting 10a will create an event titled Staff Meeting at 10AM. If you don't include an event time, then the event is created as an all-day event. That's what you are experiencing.
If you turn off Create with Natural Language in BusyCal's Settings, then new events you create will use the default start time and duration.