Because a couple of massive boulders rolled down across the highway. The first one measures 48x18x19 and weighs an estimated 2.3 million pounds. It's the one sitting on the road.
This guy hit the road and rolled just off of it into a ditch. This one is 50x50x26 and weighs 8.5 million pounds. Pictures and data from the Colorado Department of Transportation Facebook Page, where they say "rockfall mitigation will begin shortly."