why did fielding say he was sorry he had said that miss quested was mad?

The Collector insisted on an apology, Fielding had spoken out of turn and was throwing doubt upon Miss Quested's accusation that she'd been "insulted" by Aziz in the caves.

‘Miss Quested herself definitely accuses him of-' He nodded, and turned his face away.

'Then she's mad.'

'I cannot pass that last remark,' said the Collector, waking up to the knowledge that they differed, and trembling with fury. 'You will withdraw it instantly. It is the type of remark you have permitted yourself to make ever since you came to Chandrapore.'

'I'm excessively sorry, sir; I certainly withdraw it unconditionally.' For the man was half mad himself.

'Pray, Mr Fielding, what induced you to speak to me in such a tone?'

'The news gave me a very great shock, so I must ask you to forgive me. I cannot believe that Dr Aziz is guilty.'