All aikido techniques have a backup technique or motion. Many of the techniques in kata are explicitly paired with another technique because one technique will serve as a backup in case the other fails. For instance, in junana, gedanate is the backup technique for gyakugamae .
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Ushiroate, however, is a different sort of thing. It is actually a very robust, general-purpose backup to most of the techniques in junana. It tends to happen when uke is falling on tori or pressuring tori or resisting another technique.
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Because of the general utility of ushiro as a failsafe in aikido, it could almost be considered a principle of aikido instead of technique - just like I wrote earlier about shomenate. In fact, there seems to be an analogous relationship between shomenate and ushiroate similar to the irimi/tenkan omote/ura relationships in aikikai aikido.
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Ushiro often feels to tori like swinging around a pole and if tori continues this rotation then the pull becomes a chest push as a separation. So, shomenate (on the chest) is the backup technique for ushiroate.
