What causes a broken RPMDB ??

[Erwin Rol <mailinglists erwinrol com>]

Today's update seems to have seriously broken my rpmdb. I can not
execute any rpm command anymore. 

[root xpc rpm]# rpm -qa
rpmdb: PANIC: fatal region error detected; run recovery
error: db4 error(-30977) from dbenv->open: DB_RUNRECOVERY: Fatal error,
run database recovery
error: cannot open Packages index using db3 -  (-30977)
error: cannot open Packages database in /var/lib/rpm

I could fix it by removing all __db.??? files and running rpm
--rebuilddb. But what i am wondering is what caused the corruption. I
can't find anything in the logs (like filesystem corruption) that
indicates a failure of my hardware. 

 - Erwin