Thanks for query,
cause is not known but may be due to vagal stimulation while micturation.
Vagal stimulation and getting up there is vasodilation
causing sudden fall of BP.
There is no specific treatment but go for following,
Do not stand up suddenly but gradually and before standing sit on the edge of bed and after some time stend up and go for urination.
Do not do fast urination but slowly,
Consume little amount of alcohol.
not to be taken as it may cause bradycardia
and during syncope
also there is bradycardia.
Ok and bye.