They're tracking any open source project that's registered on their website. If you have or know about an open source project that's not on Ohloh, you'll have to register it and specify its repositories (and branches).
Well, then a possible explanation is that people don't submit much projects to their website anymore and thus there are only "old" projects that are getting more and more mature and thus needing less commits.
I couldn't find any figure to back this though.
But I do remember that Ohloh used to be quite popular and I'm sure as hell that it's not anymore.
Ohloh shows scary decline in open-source project additions to Ohloh
That the most probable explanation, I believe. But did you find a real figure somewhere? I couldn't.
I wondered what exactly was being measured here too. I checked around the site quickly but found no answers. Bah. Might as well be made up numbers as far as I'm concerned.