Three mechanisms overlap. charm forces hedgers to adjust deltas as expiring options decay; pinning pulls price toward strikes with heavy open-interest; and once the expiring contracts are gone, the dealer-gamma that was damping moves disappears with them.
That last point produces the most cited pattern: the week after a large expiration often sees wider ranges, simply because a stabilising hedging flow has been removed. The effect is statistical and modest, and it is routinely overwhelmed by news.
Example: XYZ grinds in a $0.40 range all week and settles at $50.02 on the third Friday with 30,000 contracts at the $50 strike. The following Tuesday it moves $1.80 on no obvious catalyst. Nothing changed about XYZ; the hedging that had been absorbing flow expired.
Related: opex, pinning, dealer-gamma, triple-witching