Why does my cat do this?

We have our house for sale and once in a while we have open houses or people come and look at it. Well both of my cats are afraid of people (other than the people who live here) and so they obviously don't like this. One of the always pees on clothes or other stuff after someone looks at our house. The only other time he peed somewhere other than his litter box was when we first got him a few weeks ago. Why does he do this?