The startle caused you to eject some fresh acidic piss from your bladder that stopped and stayed in your urethra.
Back in more primitive times, when we had an encounter with a predator that shocked us, our automatic reflex was to piss ourselves because the pheromones in our urine scared the predators off.
The social stigma of pissing ourselves has meant our mind has taken over this reflex and stops it in its tracks (not babies though as they have yet to be socially conditioned in this way). It's also why kids piss the bed way more than adults, it's because they're having a nightmare which triggers the same response.
