This unusually complex way of adding a suffix can be explained by its reconstructed origin: a voiced palatal fricative.

Palatal fricatives are rare phonemes, especially the voiced palatal fricative, but they occur somewhat more often as allophones.

Another etymology is the vocalization of a fricative such as the voiced velar fricative or voiced palatal fricative, e.g. Finnish illative case, or even an approximant, as the English 'r'.