Shy Bladder Syndrome or Paruresis is massively common, more so amongst men than women due to the urinal bowls or open troughs, with little or no privacy screening, which men are expected to use en masse.
To quote from Wikipedia, ‘It appears that paruresis involves a tightening of the sphincter or bladder neck due to a sympathetic nervous system response.’
Whilst you may feel fairly relaxed, your inner animal brain may feel less so. Standing with your cock out whilst strangers stand nearby and move behind you is not the safest position. Imagine you were a Wildebeest on the plains of Africa, having a pee, then you spotted in the distance what might be an approaching Lion. Would you:
a) Hope it at least let you finish your pee before attacking
b) Start running awkwardly whilst still leaking pee everywhere
c) Stop peeing instantly and prepare for fight/flight/freeze
Your animal brain favours c) so if you feel at all anxious/threatened, e.g. self-conscious that the other guy is wondering why you’re not peeing like him, your animal brain locks up the sphincter and refuses to cooperate, whatever your conscious brain is saying.
There are several good books on Amazon on Shy Bladder Syndrome, but overcoming a longterm spooked animal brain system, with the performance anxiety of previous failures loading the odds against you, is not easy.
Perhaps best to scrap these awful male urinals or else install heavy screening between them, as is becoming more common as understanding of SBS grows.