It is perfectly normal for the other door to open and close due to the air pressure inside the refrigerator.
There is a path for cold air to travel between the freezer and refrigerator. When one door is opened and closed, the outside air and the pressure inside the fridge are pushed to the other side, and this can push the other door to open up a little and close back.
If the freezer door remains open, then you should:
Check the levelling legs: The front of the refrigerator should be slightly higher than the back to allow the door to close normally. When properly adjusted, the door will close smoothly and firmly on its own. To know how to adjust the leg, watch this video.
Check to see if the refrigerator door gasket is sealing properly: All refrigerator door gaskets are designed to rub against the face of the refrigerator as it closes. If the gasket is dirty, it might restrict the door from closing. Keeping the gaskets clean and lubricated prevents them from dragging and restricting door closure.
Distribute the weight: Place heavier items in the freezer door shelf, since this is the smaller door and is more likely to pop open. Make sure there is nothing in the freezer or refrigerator that is preventing a door from closing tightly.
Ensure lubrication: Another hack around this is to apply Vaseline or petroleum jelly on the gasket, it can soften the gasket to help form a good seal around the door of your refrigerator.