Ex-dividend date https://en.wikipedia.org/wiki/Ex-dividend_date
Adjustment of Standing Orders
On some markets, after the close of business on the day before the ex-dividend date and before the market opens on the ex-dividend date, all open good-until-canceled limit, stop, and stop limit orders are automatically reduced by the amount of the dividend, except for orders that the customer indicated "do not reduce." This is done because the dividend payout will decrease the value of the company, as it comes directly from the company's reserves. At the market opening on the ex-dividend date, the stock will trade at a lower price, adjusted for the amount of the dividend paid. If a corporation is distributing something other than a cash dividend, such as rights or warrants, then the relevant date is called an ex rights date, or ex warrants date, etc.