Web users have certain expectations about what will happen when they tab onto a button, click in a form field, or select an item from a combo box. These sorts of behaviours are historically one step in a process in which the user first gives focus to the item (e.g., they enter a form field), then they make a selection, then exit the form field, and ultimately submit the form after they’ve made their selections.
If any of these steps prematurely triggers a change of context such as opening a new window, moving focus to a different component, going to a new page, or significantly re-arranging the content of the page, this can be very disorienting for users, especially screen reader users, screen magnification users, and other users with disabilities.
