Open orders can be cancelled in the event of a stop out.
Related orders can be cancelled if the related position is closed as a result of another order.
An open order can also be cancelled in the event of a Corporate Action.
Corporate Actions such as stock splits, reverse stock splits, bonus issues, mandatory mergers, spin offs, ticker changes and delistings will all result in existing trade orders being cancelled.
A Corporate Action, such as a cash dividend, stock dividend, optional dividend or rights issue, may result in an order cancellation if the change in the price of the security exceeds 20% due to the Corporate Action.
This applies to both related and standalone orders in Single Stocks and Single Stock Options.
You can find additional information regarding various corporate actions here.