By 0:02, Sagan is on the midline of the road, Demare is on his right - almost but not quite drawn level with him, and Cavendish is following Demare - not quite as far to the right, with his wheel perhaps 30cm to the right of the centre line.
By 0:04 (just after they've reappeared from under a tree), Demare has got just ahead of Sagan, and Cavendish is following directly behind him, less than a bike length behind. All three have moved further to the right, as have others in front of Sagan.
By 0:05 (reappearing after another tree), Demare's rear wheel is now fully clear of Sagan's front, Cavendish's front wheel is roughly level with Sagan's rear, and Sagan has a clear space in front of him - he can go straight ahead. Cavendish doesn't have any more room available to his right, but he does have enough to go straight on behind Demare.
By 0:06, Sagan is still moving right. Cavendish's front wheel is level with Sagan's frame, and all he could do to avoid Sagan is to brake violently. Demare has started moving to the left. Then the next tree obscures them, under which first contact happens.
I think it looks like Sagan seeing there's now a gap to the right of Demare, and moving towards it while ignoring the rider to his right - and bracing himself for the impact he's going to cause. He probably hadn't thought about whether Cavendish had any room to avoid him, but he was expecting contact due to his move.
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