Implications of the continuity of Currier "languages"

[MarcoP]

But in concrete terms, what is it exactly that causes this gradual change? Vocabulary? Or the increasing and declining use of glyphs/clusters in specific positions (kind of like morphemes)?

The change can be observed both in vocabulary and in "morphemes". 
For instance 

• the word 'chol' and the 'cluster' sho are typical of A
  
• the word 'aiin' and the fragment 'dy' are typical of B
  

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The two aspects are of course related: -dy words tend to be more frequent in B than in A.
On the other hand, the 'cluster' aiin has more or less the same frequency in A and B, but as a stand-alone word it is about three times as frequent in B than in A.