Question

what was the causes of the defeat of Siraj Ud daulah in the Battle of Plassey

Answer 1

The main cause of defeat of Siraj-ud-Daulah was the betrayal of his army by Mir Zafar, one of the commanders of his army. Mir Zafar defected to the British with a major chunk of Siraj-ud-Daulah's army causing the defeat of the Nawab.

Answer 2

Robert Clive bribed Mir Jafar, the commander in chief of the Nawab's army, and also promised him to make him Nawab of Bengal. He defeayed the Nawab at Plassey in 1757 and captured Calcutta. ... Tensions and suspicions between Siraj-ud-daulah  and the British culminated in the Battle of Plassey.