Were you able to get this figured out? Sometimes there is one date out there on a task or issue and if the task constraints are following that, everything gets bumped out.
To me if the duration changes that dramatically you must also have some predecessors in place and when you update the task constraint it changes the dates out as well. If you make yourself a view where you can see the task duration and the predecessors for each task at the same time, it might make things a little clearer for troubleshooting.
That view is actually my default now when setting up projects because so many things are affected by those fields.