Google announced a new reporting tool to see how your campaigns are moving in terms of changes to your budgets and clicks.
In addition, when possible, Google will try to explain why the change happened. At least, Google will try to give you one possible explanation.
Google explained that the top movers report was designed to show which campaigns and ad groups have experienced the largest changes in clicks and cost. The report will also highlight changes you made which might have contributed to those moves. Google added that the "top movers report will look inside your campaigns to highlight big moves that might have been easily overlooked."
Here are some example reports:
Forum discussion at Google+.