Other factors have entered the picture specifically down there. You have to look around and see if any structure, tree or otherwise that has wind block potential has come down. This would load the tree in question with more wind than ever previously. A tree community works to share these loads. If that structure (usually tree) was touching 'your' tree always or only part of the time (during extreme loading), it was probably giving support. The farther from the pivot of the ground, the more support.
So, if a nearby tree just came down, and they were touching; your tree just increased wind loading and decreased support in a single 'move'. This alone can cause problems. If it also has weaknesses/ codoms; those factors can compound on each other and give later failure.
If lots of water in ground recently, need to determine loaded axis of the tree, and make sure roots aren't pulling out of the ground opposite lean, and/or ground sinking on the lean side at base/pivot (where forces are double loaded). 2 trees groan together at ground is codom to me; each preventing the other's roots from gaining leveraged distance opposite the lean as 1 codom branch does to another. Also, each half trying to keep the other from being round/ giving levrageable axises; if anything giving the cross axis to the loading more leverage, just as codom branchings do. Also, pushing each other apart in later growth as codoms do. Then, plus possible soil loosened from rain around these codoms at the ground connection; to further compound failures, of codoms connected at ground level.
But, after a while; you will have all survivors; with many of the sick and weak thinned from the herd. But always watch for changed conditions a tree is incurring; it's local history is important.
