org-clock-out-switch-to-state

org-clock-out-switch-to-state is a variable defined in `org-clock.el'.
Its value is

nil

Documentation:
Set task to a special todo state after clocking out.
The value should be the state to which the entry should be
switched. If the value is a function, it must take one
parameter (the current TODO state of the item) and return the
state to switch it to.

You can customize this variable.