plugins mysteriously deactivating

on our wpmu server (wp core version 2.9.1, using subdomains) several people reported that one of their plugins mysteriously deactivated yesterday. the plugins involved so far:

custom css

there are 3 blogs reporting this, and each one had one of the above plugins deactivate on them. other blogs using the same plugins did not have theirs deactivated. the problem blogs all had several active plugins, but only one deactivated. none of these plugins have been updated recently, and there is no overlap in who has user accounts on these blogs (other than the admin user). our netsys guys say they haven't touched the server. the only things i can think of that i did at the site-wide level in this timespan are: 1) updated DM Albums via automatic upgrader (which none of these blogs were using). 2) added plugins TinyMCE Advanced and Youtube Widget (which none of these blogs were using). 3) created several new blogs. any ideas? i'm failing to see the pattern. please let me know if there's any more information i can provide that would be helpful. thank you!