Box Office Flops

Hollywood is starting to make excuses after box office totals dropped again this weekend. Basically every big movie is opening up to huge numbers and then losing a significant percentage of the audience the next weekend. So far this year the box office totals are not adding up to what was predicted.
My guess - when a big movie opens, most people that want to see it go out and see if the first weekend. After that, people wait for DVD. I don't get why Hollywood is so alarmed by the drop offs and why they would predict much more. When you get to a third movie in a trilogy you are only going to get an audience that liked the first two, you will probably not get any new viewers. So if you liked the first two you're going to go the first weekend to the new one.