Today (02-November-2013) loose rock did fall down at popular hiking router in Thingvellir national park in Iceland. The rocks that did drop have estimated weight of around 2 tons for the largest ones. No damage took place and nobody was hurt following this rock slide. While not common it does happen that rock fall down in Thingvellir since frost and water slowly crack the rock and make it loose with time.
There is a reaming risk of more rock falling down in this area. When they might come down is impossible to know for sure. Currently the employees of the Thingvellir national park are estimating the risk of more rock coming down at this location.
Icelandic news about this