Because anyone holding puts or calls at that strike will be motivated to sell them as there is no intrinsic value in them....if they held them and price never moved...the would just decay until worthless. Sellers obviously like that. By the way....is anyone else finding these pages taking a long time to load on this site.
The window I use for Max Pain runs from the Thursday prior to the week of expiration into expiration Friday. In non-trending markets, Max Pain is usually satisfied on Mon-Tuesday the week of expiration. In trending markets, one end of the window will often hit the level of Max Pain.
Once or twice a year Max Pain will not be fulfilled in the above window. That's often times in very strong trends. May 2003 is such an example.
I'm just sharing my observation of how it usually progresses. I am not as certain as to how or why it works.
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