Why $ionicView.beforeEnter is called twice in ion-tabs ?(With Plunker attached)

I have tried to add ion-tabs at the top of a page(HomeTabPage), but I found that the $ionicView.beforeEnter is called twice.
Is it because the two tabs are sharing the same ctrl? Please see this Plunker.

But if I separate the two tab views into different controllers, the first tab beforeEnter event also called twice.

BTW, if I do not add the vm.goToSecondTab(), the second tab view won't show.

Why this happens? Can anyone please help explain a bit?

Thanks very much!