Huntington Disease Anticipation

A 35-year-old man presents to the neurology clinic with progressive involuntary movements of the face and limbs over the past 2 years, along with declining work performance due to difficulty concentrating. His father died at age 42 with a similar presentation. Genetic testing confirms the diagnosis of Huntington disease. The inheritance pattern marked **B** in the diagram shows autosomal dominant inheritance with anticipation and paternal expansion bias. Which of the following molecular mechanisms BEST explains why this patient's symptoms began earlier and are more severe than his father's?