Excess pattern repeats lead to strand slippage/errors due to an unstable region (in this case, excess Cytidine bases).
It could be a repeated pattern as well (ie the trinucleotide repeat CAG in Huntington's).
here's a more in depth explanation (from wikipedia article on Slipped-strand mispairing):
A slippage event normally occurs when a sequence of repetitive nucleotides (tandem repeats) are found at the site of replication. Tandem repeats are unstable regions of the genome where frequent insertions and deletions of nucleotides can take place, resulting in genome rearrangements.