markerTapBehavior property
The default MarkerTapBehavior is MarkerTapBehavior.togglePopupAndHideRest which will toggle the popup of the tapped marker and hide all other popups. This is a sensible default when you only want to show a single popup at a time but if you show multiple popups you probably want to use MarkerTapBehavior.togglePopup.
For more information and other options see MarkerTapBehavior.
Implementation
final MarkerTapBehavior markerTapBehavior;