Sforno on Deuteronomy 9:8/ספורנו על דברים ט׳:ח׳

1ובחורב הקצפתם, Moses underlines the serious nature of this negative characteristic known as “stiff-necked,” by reminding the people that even at Mount Sinai when G’d had already decided to wipe them out, (verse 14 our chapter) G’d had justified this by referring to their stiff-necked nature. He had not decreed death for them on account of a specific and horrific sin, but on account of their attitude which bode ill for the future, indicated that such a people was not capable of becoming true penitents.