Hearing water running can cause a strong urge to pee, especially in small children, and that may be why that happens to you in the shower. You are not really going to contaminate the shower, it will be washed down the drain and won't hurt anything. Urine normally contains no bacteria, and the shower drain goes into the same pipe as the toilet, so no matter where you pee it's going to wind up in the same place anyway. If you don't want it to happen, the best way to prevent it would be to use the toilet just before you get into the shower.