Read: “Understanding SC 3.2.1 on Focus” by @artofvision

3.2.1 On Focus: When any component receives focus, it does not initiate a change of context. (Level A) The intent of this success criterion is to make sure that any unwanted actions are not initiated when focus moves on to an element. For example during tab navigation or shift tab navigation if user focus moves on to a link & a modal is triggered this fails this check point. Here user did not initiate this action; it was initiated when user focus moved on to a particular element.