It was the worst possible time for AS to drop a chain. The attacks were starting, and AS took too much time to get back on his bike. Maybe if he had a teammate there, it wouldn't have taken Andy so long, who knows. Where they were in the race, and on the climb, it was just plain bad luck. If it was earlier in the stage, it would have been different, and easier to wait for him.
Put yourself in the rider's position. It's hot, hard, and you're going full out. To wait as long as needed at that particular point in the race would have been too hectic and hard to control.
Who really knows. I will say that the Schlecks seem to have a fair amount of bad luck and bad timing. Personally, I was getting tired of the way that AS was racing. There was only one stage where he had AC in trouble, so all of this talk of AC being weaker doesn't make sense. From the way that AC was looking, It's possible that he would have taken some time out of Schleck anyway. Look at who he would have had with him on the descent.