You don't always. Its a natural process. But to poop the muscle (sphincter) that holds the anus closed must be relaxed. In doing so, the muscles that control urination are also relaxed, so many people will poop and pee at the same time.

And,

It takes more muscle strength to poo than pee. Therefore, you are pushing so hard (not really hard, but intently), that both will come out.

When pooing, you use almost all muscles down there, when peeing you only use a small few.