Open orders can be cancelled in the event of a stop out or if an instrument is restricted from trading.
Related orders can be cancelled if the related position is closed as a result of another order.
Please also see order rejection reasons here.
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, CFDs and Single Stock Options.
You can find additional information regarding various corporate actions here.