First did you make the trap round with no corners. If there is a corner they will climb out of the trap. I've seen them do it personally on our small trap. We had to put a top on it. Also if they can get their nose under the door they will escape that way to. I've seen them go partially in on a spring loaded trap and let the others loose. Pigs are intelligent and learn really fast. We have our door rigged with a pin that keeps the door from opening unless you pull the pin out. The hogs had learned to use their nose to ease it up and escape.