attached the timer behavior is still in the DOM document. When a component with a timer behavior is replaced in java in an ajax request by another component without this behavior, this check is not enough. In this case, the component id is still in the DOM (it was replaced by another one by the same id).
See email thread at:
WICKET-6191 AjaxTimerBehavior will stop after ajax update of component it is attached to
- is related to
WICKET-6523 Two AbstractAjaxTimerBehaviors on same component timeId conflict