While unconfirmed, it is possible that Google rolled out a semi-major update over the past weekend, as Webmasters have taken to the forums to complain about loss in traffic.
The person who started the thread says some webmasters are experiencing a 30-40% drop in traffic. Another user says they saw a 30-40% drop for a ten-year-old "authority site".
"One of my sites was hit on the 23rd.. others (ww members) the 30th... now you guys on the 6th. This update is hitting people in waves on a Thursday / Friday but it doesn't affect everyone at once. The update appears to be isolated to a selected group each time," says forum member petehall. "Wonder how long it takes for a Panda refresh to complete, as this seems to be the main connection with the 23rd of March."
That is indeed the day Google tweeted:
Other forum members chimed in to express similar issues in traffic drops.
Here are the changes Google made in March (at least the ones they have disclosed to the public). Unfortunately, we'll have to wait a while to see what all Google has done in April.
We've reached out to Google to inquire about an update over the weekend, and will update accordingly, though I don't expect much more than a "we make changes every day" kind of response.