Why are so many immigration department’s senior staff leaving their jobs?

The recent departure of numerous executives from the department of immigration is not a result of staff being sacked but rather voluntary departures, says the DIBP.
In a sharp response to an article in The Canberra Times – which reported the department was facing "the public service's greatest executive brain drain since the 1980s, with a quarter of its upper ranks either shown the door or turning their backs on the department since its takeover by customs" – the DIBP has told the paper that the story published on Friday contained "inaccuracies, incomplete information and speculation" regarding departures, and that "the secretary [Michael Pezzullo] has stated that a number of ... officers have [decided to move to other departments or] chosen to leave the organisation voluntarily".
The Canberra Times has confirmed the toll of departures, either already announced or coming up, has reached 30, from a senior executive cohort of 119 in June 2014. Four deputy secretaries, nine first assistant secretaries and 17 assistant secretaries have now confirmed their departures or are expected soon to do so. An unprecedented 10 of them have opted to follow their old boss Martin Bowles to the Health Department.
“Insiders have complained about the management style of the new regime and there has been unhappiness from veteran public servants forced to wear the military-style Australian Border Force uniform to work each day after a lifetime of civilian service.
...