Farting unintentionally while urination is occurring due to relaxation of the sphincter. An expulsion of pressure which leads to many an awkward public toilet moment.
I laughed my head off as I heard the lady in the next cubical do a squeaky one before her stream flowed. It was a belting pee fart!
I was dying to relax and pee but I knew a fart bubble would be released prior and everyone would hear my pee fart.