The cause behind the hiding of the grave of Fatimah(as) was done by her command and will, it is showing the protest to the movement that had begun after the Prophet's (pbuh) death regarding the caliphate, and the trampling of rights and the reality.1
Secondly her grave is a barrier that doesn't let the change of the history of Islam advent and prevents from some claiming that shi didn't have any protest to the circumstances then about the caliphate.
1 see: Ibn Abil-Hadid, Sharh Nahjil-Balaghah, vol. 16; Sheikh Abdullah Naser, Mihnatu Fatimah ba'da Wafati Rasulillah (s) (Nusus Tarikhiyyah Min Masadiril-Sunnatil-Mu'tamadah),pp.197-206.
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