If urination normally takes place when one is relaxed (parasympathetic
nervous system), then why does involuntary urination occur under
extreme fright (sympathetic nervous system)?

The initial flight or fight response involves initial activation of
the sympathetic nervous system. Under extreme stress however the
parasympathetic nervous system is activated in the "faint and freeze
response."

More precisely and accurately, extreme stress can initiate activity in
the VVC and DVC (ventral and dorsal vagal complexes) which activate
the parasympathetic NS, while the classic stress pathway can activate
the sympathetic NS.  These contrary signals are hypothesized to be 
the cause of "voodoo death", and can cause muscle and vasospasm (hence
involuntary micturition).

Furthermore, in cases where there is a rapid drop in blood pressure
due to fright, (VVC activation), and the simultaneous relaxing of
sympathetic tone, effects such as loss of conciousness and micturition
can occur.

One more thing, when thinking about this complex reflex pathway,
remember that the brain's output to the spine (output from pontine and
medullary micturition control centers through the gracile and cuneate
tracts) have an overall inhibitory tone on micturition.

- Patrick