A retort is a sterilizing device that is used to sterilize things of bacteria that might contaminate otherwise.  You failed to mention whether the beans cans were empty or full of beans and sealed already.  If the cans were empty when they were placed inside the retort, that might produce some concave formation, given there was an empty space within the can.  If there were some type of form that could be placed inside the can to keep the metal of the can from deforming, that might be a logical solution to the problem.  If the beans are already in the can, with the can sealed off, that might solve the problem of the cans coming out in a concave formation as well.  What has basically happened is there was a weaker force created inside the can than existed on the outside of the can, so the can was deformed into a concave shape.