When updating a package via rpm or yum, you may observe old rpm's scripts breaking install of the new one. Assume we have my-app-0.1 rpm installed on the system, and my-app-0.2 being an update. Here is how the rpm scripts are getting called when we execute rpm -U my-app or yum update my-app:
install: %pre(my-app-0.2)
install: %post(my-app-0.2)
erase: %postun(my-app-0.1) - surprise! blast from the past :-)
I did not have a script in %preun stanza in the rpm I was troubleshooting, but you should be able to debug your case with rpm -vvv -U my-app-0.1.rpm.