The information provided with the Hotfixes indicates it was a certificate issue:
DAT 6807 and 6808 introduced a new Digital Certificate which causedsubsequent Trust issues for some VirusScan installations. Symptoms of affectedmachines could vary from logged errors to missed detections. DAT 6809 or laterin conjunction with this hotfix, provide a resolution.
Why it didn't affect all installs that had the dat files installed hasn't been released, but if I had to speculate I would suggest it is something to do with root certificates and revocation checking.