I used to drive coaches to skiing resorts and it really is shocking that there wasnt snow on the road when the bus crashed.
Many factors are possible but I always felt comfortable even on snow so I guess the driver could have been distracted or something.
Bad news
I would normally state the obvious, snow and ice but according to the news it was just a bit of rain so who knows.
Maybe the driver fell asleep or was getting distracted by some poorly discliplined kid
In my schools ski trip last year the bus nearly fell in the lake, but luckily got stuck in a snow bank first (I don't know exactly how it happened.). It really does make you think about how easily it could be you. :S
I know, I went on a ski trip in America in 2010 and it was fine until the UK had about an inch of snow and closed all of the airports. We had to wait a week to get back.