There are a few things that cause this problems with one piece garage doors usually there is a metal rod that runs the length of the door and there should be one on top and one on the bottom of the garage door and in the middle will be a triangular piece of metal and this rod is tightened up sometimes they are adjustable and what this does is acts like a strut for a wood door since wood doors will tend to warp from moisture and the elements.
loose struts
Sometimes these struts can work their way loose and even sometimes the triangular metal piece in the direct middle will sometimes come away from the door and cause the rod not to put any tension on the garage door so it will tend to warp when the door is in the up position because all of the weight is on the middle of the door when the door is up other than when it is down. If you want to try and fix this problem yourself then sometimes it can be pretty easy the first thing you will have to do is make sure the steel triangle piece of metal is attached to the door if it has come apart from the door then find a way to screw or nail it back into place remember it must be directly in the middle of the garage door and then the rod will slide into a slot that you will see and then if it is not tight you will need to tighten up that rod on the left or right hand side of the garage door there should be an adjustment or a nut that will make it tighter you would just have to adjust to wear it is not sagging anymore when the door is in the up position. Now this can happen to the top or the bottom of the garage door so you should check both.
usually there is an opener attached…
No the other reason for this problem to occur would be the piece of lumber that is used in the framing of the door there is usually a 2 by 6 by 16 feet long piece of wood that runs along the bottom and the top of the garage door this piece of wood can sometimes split from years of use and or the elements especially moisture but the most common reason for them to split is usually there is an opener attached to the top piece of wood that is used in the framing of the door. And sometimes a split on this piece of wood is not always visible to the naked eye and especially if the wood is painted it can be even harder to notice until usually when the door is being opened and the opener is pulling on the wood can you see it start to split. Sometimes what will happen is the lag screws that hold the open a bracket on the top of the garage door may be too big or not have been pre-drilled when it was installed and overtime those screws can distort the wood and make it split especially if there is an open and constantly pulling and pushing on it. If you cannot figure out how to fix this problem then you might have to replace the piece of 2 by 6 by 12 or 16 feet however big your garage door is but my recommendation would be to call a garage door company unless you are in the construction business and is very good with framing of wood.