[CentOS] Automated YUM update kills DNS

We've got several CentOS 3.x systems running DNS that we keep updated automatically via YUM.

Recently two of those systems (not all of them) when updating themselves to the latest versions of BIND, automatically replaced /etc/named.conf with a new one and saved the old one as /etc/named.conf.rpmsave.

Which of course broke DNS for those servers.

All servers got updated, but only two of them did the rewrite.

Any ideas as to why it might have happed, or how to make sure it doesn't happen again?

Hopefully nothing so simplistic as "exclude bind from auto updates" though if that's the only answer, we'll take it.

Thanks.

Jeff