The rest of the bowel contents are not highly fluid as a rule. They have enough viscosity or structure to drag the bubbles along with them, even downwards. Furthermore, the bowel is very sensitive to stimulus from the texture of its contents. It can feel the difference between solid liquid and gas. An important function is to get rid of gas before it causes painful and even dangerous bloating (flatulence if you like) so where a bowel is stimulated by the presence of gas, it increases its peristaltic motion, generally massaging the bubbles towards the exit faster than either liquid or solid material.