Why does baby pee when it's loud in his surroundings?

I babysit an 18 month old boy and I noticed that when he hears me yelling at my kids during diaper change, or one of the other kids are making noises he pees! Well my kids are forever being out of control, loud etc so I am constantly yelling at them! Well sometimes this happens during baby's diaper change and... show more I babysit an 18 month old boy and I noticed that when he hears me yelling at my kids during diaper change, or one of the other kids are making noises he pees! Well my kids are forever being out of control, loud etc so I am constantly yelling at them! Well sometimes this happens during baby's diaper change and he will literally pee on me! Is it a coincidence or do they pee like out of being scared or nervous like a dog does? I did notice both times he peed on me I was yelling at the kids for something! And once he almost got me and I wasn't yelling but my kids and neighbor kids were. Odd
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