It's definitely some kind of filter. One can verify that by searching "site:example.com keyword" or "brandname keyword" which both bring up the regular page while it's completely removed from the SERPs if you are searching just for "keyword" (the first result from the domain example.com is now some different page which is not part of that subdirectory).
Btw, the subdirectory which is affected was created 2004 and ranked well for many years. For one and two keywords the results are filtered out (it a filter because they are completely removed - it's not about being passed by competitors and going down a few positions e.g. one page was #4 for a single keyword and it's now not within the first 1000 results). The remaining traffic (25%) is from long tail searches which are not affected.
Anyway, of course I try to overcome the filer - it's more important than understanding it. (My guess is that it might be related to this discussion: Google to Target Overly SEOd Sites [webmasterworld.com]) However, so far changes donít solve the problem.