When a Planter or Window Box-wall begins to bulge out, it is typically due to a drainage issue. (see How does my planter drain excess water?) Please check the underside of your Planter/Box to ensure that nothing is blocking the drainage hole. If you find that there is a blockage, go ahead and clear the hole using something long and pointy (a coat hanger wire or something to that effect, should do the trick). Water should then be expelled through the drainage hole.
The material Mayne products are made from has an incredible memory, so after the water drains from the planter, it will slowly return to its original form. Leaving the window box or planter out in a warm, sunny area can help to speed up this process!
If this method of clearing the blockage is unsuccessful, the easiest solution to the problem would be to drill another small hole in the backside of the planter/box for extra drainage. This option is also recommended for those who may have their boxes situated in an area that receives a moderate amount of rainfall.
Another potential cause of a budging planter may be a plant that has a large root system (see What kind of plants should I plant?).