As contracts expire, the hedges dealers hold against them are no longer needed. A large block of open-interest at one strike can hold price near that level through the morning — see pinning — and then release it once the contracts are gone.
Traders use opex mainly as a context flag, not a signal: ranges compress into it and expand after it, and positioning-based estimates such as gamma-exposure reset the following Monday.
Example: XYZ sits at $50.20 with 40,000 contracts of open interest at the $50 strike and under 5,000 at every other. Price grinds in a $49.85–$50.25 band all Friday, then gaps to $51.40 on Monday with no company news. Nothing changed except that the hedging demand expired.
Related: pinning, triple-witching