What about the 20 years between 1990 and 2010? The 25 years between 1990 and 2015?
I'm not saying they didn't happen, but they seem to be happening much more frequently now. Hence why we actually have groundings to see what's going on.
I really think it's a cultural issue. People get lazy, think technology/training will do it for them, and then reality hits. Hard.
Edit: Or, I guess, pilot error isn't the cause? If it is pilot error, though, then maybe my theory has some credence?