The most common reason for a stuck closed opening of the urethra is an excess of semen
How the semen gets there is largrly immaterial, but when it dries inside the tip of the penis it can create an unfortunate study in hydrualics.
This can also cause embrassing liquid stains. No need to elaborate.
The dangers in a public restroom are too awful to mention, but suffice it to say that most men stand very, very close to the urinals.