Blogs getting marked as spam on wordpress multisite

We have recently started a blogging (WP multisite) + buddypress community in December at
Since last 3 days we are facing a strange problem. The blogs created by the admin are automatically getting marked as spam.

We basically use several blogs (subdoomains) for our own services like for audio content, for forums etc. These blogs that are created by us get marked as spam sporadically. It has happened 4 times in last 3 days. We have to unspam those blogs to make the services available again.

We have checked member privileges and there is only one network admin and no one else has privileges to mark blogs as spam.

What could cause this?