Motivation
The second palatalization of velars is a direct consequence of the monophthongization of diphthongs, or more precisely, the change *aj > ē. While the clusters *kaj, *gaj and *xaj were in accordance with the principle of so-called intrasyllabic synharmony that operated during the Common Slavic period, the clusters resulting from monophthongization *kē. *gē, *xē defied the intrasyllabic synharmony because the velar consonant was preceding the front vocalic, and that defied the Proto-Slavic phonotactical constraints.
That anomaly has been resolved by palatalization of velar consonant, just as it was done during the preceding first palatalization. Only the results of this new palatalization were different, and not completely uniform on all Slavic territory, indicating first dialectal differences. Usually this palatalization is described as gradual, first fronting of the velars to proper palatals occurred, and then (perhaps with those that were affected with the third palatalization) they were assibilated. Hence it's sometimes called sibilantization.
In addition, the same process operated before the new instances of *i deriving from *oj.
Read more about this topic: Slavic Second Palatalization
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