I wonder if it is the intensity with which riders are training, possibly combined with a greater focus on numbers. I know that when I was training for some good times in TTs, or at the point when the elastic is about to snap on a club run, my perceptions are reduced (e.g. just to the wheel in front) and so I am less likely to be aware of potential dangers. (That's one reason I take things a bit easier now, except for the annual club hill climb)
The other factor, IMHO, is that racing is much more competitive now. 20 years ago, the first two hours of most stage races (except the TdF) was a case of piano, let the break go down the road. Now teams are trying something every hour of every race. And everyone has access to top-level science, so they are squeezing more performance out of every rider - again that puts pressure to train to the margins, learning to descend faster, getting more aero on a TT bike, taking a more aggressive line through that cobbled turn....
