Many reasons. The simplest thing is that the packaging technology is not skilled enough. If the stuffing is not put enough and the middle is pressed too deep, grasping the bottom of the steamed stuffed bun for a long time will make the top or edge of the steamed stuffed bun thin. Of course, it is easy to explode when steaming, or there is a problem when steaming. Steamed buns should be steamed to a certain extent, otherwise the buns will explode, just like you said. If the steam is not strong, deflate for 4 minutes before steaming for about 7 minutes every 1 minute (professional steamer or steamer).