I had to look this up, but here’s a paper I found that touches on this: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7105313/
The TL;DR is we’re not quite sure, there are multiple different possibilities, and the exact mechanism probably differs from case to case. In general, repetitive sequences open the door for a lot of weird things to happen, like the DNA folding over on itself to form a hairpin, or temporarily forming a triple-stranded structure, or even just “slipping” and adding/removing sections during replication.
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