Syllabic Synharmony
After the progressive palatalization took place, a tendency arose in the Common Slavic period wherein successive segmental phonemes in a syllable assimilated articulatory features (primarily place of articulation). Another tendency, generally referred to as the "Law of Open Syllables" marks the beginning of the Common Slavic period in which an arrangement of phonemes in a syllable (from lower to higher sonority) led to final consonants being deleted, consonant clusters being simplified (either by deletion or epenthesis), diphthongs being monophthongized, nasal consonants in the syllable coda becoming the nasalization of the preceding vowel (*ę and *ǫ), etc. After these changes, a CV syllable structure (that is, one of segments ordered from lower to higher sonority) arose and the syllable became a basic structural unit of the language. Thus syllables (rather than just the consonant or the vowel) were distinguished as either "soft" or "hard;" most consonants having developed palatalized allophones in soft syllables (a situation dubbed "syllable synharmony" or the "syllabeme").
Read more about this topic: Proto-Slavic, Historical Development