How come when you urinate just but have to defecate too?

While it is possible to feel the need to urinate and defecate at the same time, it is not必ずthe case that if you urinate, you will also need to defecate. The urge to urinate is triggered by the bladder becoming full, while the urge to defecate is triggered by the rectum becoming full. These two processes are not necessarily linked.