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On page factors are also OK , it can be due to over optimization of keyword through link building (this can be true for first case but still I need some concrete stance and what can be done to remove the same) but second case is not at all understood.

I don't think this is domain crowding , DC occurs when front page lists more than 2 separate listing in the serps , but this is proper structure of a website shown with help of xml (if am not wrong) , so Google must be pulling that data from somewhere and structuring the same dynamically ( or grabbing it from sitemap) am not sure abt that .... that's why I posted this question.  At first I thought that this is happening for very high traffic and reputable websites(Google is listing top pages of the website as tabs) , but this is not the case , one of my clients website ( which is quite new with 25 domain authority and no alexa rank) also shows in the same manner on its anchor keyword, maybe someone who has actually dealt with it can give a better answer.