market sell-offs common late day due to margin?

a couple of people mentioned this phenomenon (if it is one) in different forms do margin calls go out later in the day? is that why we often have such horrible action in the last hour when the market has been weak recently?? for whatever reason, i would have thought margin calls go out after the close and then the selling is done on the open. anyone have any idea? it’s funny how both people mentioned it to me in the last few days, but i’m not sure they really “know”. thanks in advance!!!

Margin calls go out after the close, typically you are given 3 business days to settle it, so if you have gotten you margin call after Monday close, you had to settle it on Thursday before the close. Some people wait till the very last moment, especially in the current volatile market, but when a little time is left before the close and it looks like the market is not moving in your direction, you settle your margin call and that exacerbates the price action.