One theory: programmer utilization at companies has increased with the recession, so there is less time for open-source commits.

Another theory: a huge fraction of open-source in recent years is written by programmers paid by companies to work on it. The recession has caused these programmers to be pulled off and put on projects where their time can be billed directly.

Third theory, there is more use of distributed version control "offline", which would not be detected by Ohloh.