Watching the slo-mo replay I see two Sky riders veering right to avoid the first crash, and one going on to the grass verge to the left. The second of the two who veer right hits an Astana and the two Sky and the Astana go down, so I guess one of the Sky riders is Porte. Viviani was well up the front and probably didn't see. The third Sky rider stops on the verge under the tree, so it looks as though at that point there was a spare bike available, but he may then have carried on as he doesn't seem to be standing by the road in subsequent shots - at 1:18 there's a Sky rider disappearing out of shot in front of the Astana team car.
Or we could be over-analysing it.