Injuries in aikido kind of fall in different categories.
1) Sheer bad luck (these have nothing to do with aikido itself, like slipping in the showers etc.)
2) Nage is to rough and enforces the technique lacking finetuned technique and compensating with a lot of force (This is in my experience the most common reason for injury)
3) Uke being inexperienced and moving without understanding of the technique (this actually can be split into 2 categories again: A) Uke is just new and still has a lot to learn, something that nage should take in consideration. B) Uke trains for years already but doesnt understand or doesnt want to understand what is going on. These kind often also plays the badass nage)
4) Uke is moving to fast. Like Klickstein said "If you can only do ukemi at 7mph dont attack at 70mph!"