What is calling this sub other than a button click?
Nothing.
It's the Click event routine for the button btnAction.
B4A is event driven.
Each object has properties (like Left, Top, Width, Height), mehtods (like Initialize, RemoveView) and events (like Click, LongClick).
Not all objects have the same properties, methods and events.
Depending on user actions, events are raised by the operating system and the corresponding event routine is run (like the btnAction_Click routine).