plymouth-branding-openSUSE (indirectly) calls mkinitrd in %post:

( 15/353) Installing: plymouth-branding-openSUSE-13.2-10.23
..........................................................[finished]
Additional rpm output:
Creating initrd: /boot/initrd-3.17.1-1.g5c4d099-desktop
Executing: /usr/bin/dracut --logfile /var/log/YaST2/mkinitrd.log --force
--force-drivers "processor thermal ahci ata_piix ata_generic piix
ide_pci_generic fan jbd ext3 md raid1 edd"
/boot/initrd-3.17.1-1.g5c4d099-desktop 3.17.1-1.g5c4d099-desktop


# rpm -q --scripts plymouth-branding-openSUSE
postinstall scriptlet (using /bin/sh):
[...]
   if [ ! -e /.buildenv ]; then
     /usr/sbin/plymouth-set-default-theme -R openSUSE
[...]


"-R" means rebuilding the initrd.

Please do a separate mkinitrd call in %posttrans to avoid that mkinitrd runs
multiple times. (You can probably steal the %post and %posttrans script from
the kernel-firmware package ;-)