It's not always overtraining or excessive ukemi. One student I know of was kicked in the head during her 5th kyu test. She was leaning too much. It caused a blood clot which led to a stroke and brain damage. This happened in a very tame dojo by the way. Another student I know of was warming up doing back rolls when a beginner rolled into her. She wound up paralyzed. Another sad part of that one is that her husband is an Aikido instructor and may even have been teaching that class.
In my own case, I once wound up landing on the back of my neck. My neck went pop-pop-pop just like a chiropractor working on it. I still feel that injury a couple of years later and consider myself fortunate that I can still walk.
I also had a knife stuck in my shoulder in what I consider to be the stupidest thing I've ever been a part of. Not sure I could have avoided that one unless I'd walked out of the dojo and at the time wasn't grounded enough to have done so.
Sometimes, I wonder how people survive to old age.