Question

what did the mughal emperor agree to support the rebels

Answer 1

Mughal power rose to the limelight in India when British power  was at the stage of establish their rule in India 

Answer 2

Hii ..that's your answer....
The sepoys from rode all the way from Meerut and came to Red Fort where Bahadur Shah Zafar lived. Sepoys persisted him to pave the way for mutiny and proclaimed him their leader. Bahadur Shah Zafar was an ageing king. He had no other option of not agreeing to the mutiny. Also Mughal dynasty ruled over a large part of India. At some places rulers had to flee as they were threatened by British. An attempt of mutiny was also in favour because if they won the empire would be again able to maintain a strong hold over India.