The expression in isTrue is re-evaluated every time any value used in the expression changes. When the results of the expression change, it triggers a re-rendering of the component. Use aura:renderIf if you expect to show the components for both the true and false states, and it would require a server round trip to instantiate the components that aren't initially rendered. Switching condition unrenders current branch and renders the other. Otherwise, use aura:if instead if you want to instantiate the components in either its body or the else attribute, but not both.

<aura:component> <aura:renderIf isTrue="{!v.truthy}"> True <aura:set attribute="else"> False </aura:set> </aura:renderIf> </aura:component>