Cracks are a mysterious critter, but they can be attributed to several "flaws". One is a poor foundation that was not properly prepared or not thick enough or not enough rebar or improper "mix" of concrete and its ingredients. Another one is, since you mentioned it is, live tree roots will swell up and cause the slab to rise a bit, that root is powerful, look at some concrete sidewalks by a huge tree and you will understand. When the patio swells up, and the tree root dies or rots out, the slab will fall back to more than it rose. This will cause concrete to stress and crack.
Then you might have a water spickot (sp) on one side and that side could be moister than the other side and that can cause water/soil molecular density problems and cause one to fall and the other to rise.
Now remedies, live with it, tear it up and rebuild only for it to happen again, or get some "latex" concrete crack repair that will give or become more flexible than concrete since it has latex in it, and patch the crack. I truly have found that this product will work. I have a moving driveway and I use this on that major crack and it does last for several years then I dig it out and re do it.
Cracks in foundations, walls, ceilings, doors and the such are a common and usual complaint among us home owners. Learn to deal with them and live with them or else they will cause you mental fatique problems.