Pee during handjob??

I was giving my boyfriend a handjob and he was nowhere near orgasming. I was massaging the head and pee came out?? We were both pretty shocked and caught off guard. It was urine yellow and warm. No urine smell though. So wtf? Why did this happen? I thought guys couldn't pee with an erection. He was very embarrassed but I assured him it was no big deal. He said it didn't feel like an orgasm and he couldn't feel it coming whatsoever.