Recall Holiday-Whitehouse Model of DNA recombination (animation)
Consider a pair of
chromosomes, each with a (CAG)4 repeat
regions of homologous strands align "out of register"
Recombination at the "nick" produces a (CAG)6
and a (CAG)2 chromosome
Further slippage in the (CAG)6 chromosome
lead to chromosomes with multiple (10s ~ 1,000s) of repeats.