How did consonant clusters change from Sanskrit to Middle Indo-Aryan languages?

In the transition from Sanskrit to Middle Indo-Aryan languages, consonant clusters underwent significant simplification to reduce the phonological complexity of Old Indo-Aryan.1

The primary methods of modification included:

• Assimilation: In many dialects, particularly in the west and northwest, one consonant in a cluster was assimilated to the other, resulting in geminate (doubled) consonants. For example, Sanskrit sapta ("seven") became Prakrit satta, and aṣṭa ("eight") became aṭṭha.12
• Vowel Insertion: In the eastern dialect area, there was a tendency to insert a vowel within consonant clusters to break them up, rather than assimilating the sounds.1
• Specific Cluster Shifts: Certain clusters underwent standardized changes, such as the transformation of -ty-, -dy-, and -dhy- into -cc-, -jj-, and -jjh-, respectively (e.g., Sanskrit adya becoming ajja "today").1
• Vowel Shortening: The modification of these clusters often led to the shortening of preceding vowels, resulting in the emergence of short ĕ and ŏ sounds that were generally absent in Old Indo-Aryan.1

These changes were part of a broader trend toward simplifying the phonological structure of the language.1