As the Evening Standard puts it:

Tim Parker stood down as chief of staff when Mr Johnson stripped him of his role of running Transport for London from next month.

Mr Parker is the second of Mr Johnson’s deputies to leave since May and the third senior aide to go. Deputy mayor Ray Lewis was forced to stand down after wrongly claiming he was a magistrate and senior political strategist James McGrath quit over a race row. The appointment of Mr Parker, 52, one of the City’s most successful private equity businessmen, was seen as a major coup for the new City Hall administration. He was brought in with a brief to restructure the Greater London Authority and was expected to do the same with TfL’s £6.5 billion budget. But the Mayor said he was taking personal charge of TfL, which he said was “absolutely crucial” to doing his job effectively. He said the nature of the decisions TfL made were “too political” for an unelected aide to preside over.

All of which raises the question, why didn’t Boris Johnson notice previously what sort of decisions TfL makes? It’s not as if it’s a low-profile or peripheral body after all.