Why did the Soviet's attack? 
 Sorry, I have no other information about the March Days massacres. I can tell you that in the wake of the demise of the Safavid Empire, Baku and its environs were briefly occupied by the Russians and consequence of the Russo-Persian War of 1722-1723. 
 By March 1920, it was obvious that Soviet would attack Baku. 
 Viadimir Lenin said that the invasion was justified as Soviet Russia could not survive without Baku's oil. By the way, Baku is Azerbaijan capital city. 
 Is oil an important commodity there?
By March 1920, it was obvious that Soviet would attack Baku.