The queen and her performance may be one of the many stimuli for swarming but
the workers are definitely in control.  I once watched a swarm issue from a
mini nuc.  Bees were in the air swirling round in the traditional manner.
The queen was on the board the nuc was sitting on and she did NOT want to go
although she was being thoroughly jostled by her daughters.  I managed to
herd her back into the hive with my fingers and close the QE entrance.  A few
hours later when all was quiet I removed the queen for introduction to
another hive and left the solitary Q cell which later produced a very
satisfactory queen.
In this case the stimulus was clearly over crowding.
Chris Slade