The rumours were quickly dismissed by the AoT board chairman, who insisted Mr Makin's decision was voluntary.
"He was not forced by me or the AoT board to resign," said chairman Prasong Phunthanet, who is also chief of its Revenue Department.
Mr Prasong said he only learnt about Mr Makin's decision yesterday when he found Mr Makin's resignation on his table when he arrived at his office for work.
"We've never had work or personal conflicts with him. We've got along well so far," Mr Prasong said.
He said he phoned Mr Makin to inquire about his departure, but said he could not ask Mr Makin to change his mind because it was not the first time he had attempted to leave the AoT, a public company that manages several of the country's airports.
Mr Makin told him he had finished planning AoT business strategies and now wanted more time to run his own businesses, Mr Prasong said.
Mr Makin had asked to resign two months earlier after Mr Prasong gained the AoT chairmanship.
Mr Prasong said he had assumed then that Mr Makin intended to show his working spirit in light of the management change.
He said he would discuss Mr Makin's resignation with other board members today. Mr Makin, whose resignation takes effect on Sept 16, is also scheduled to attend the meeting, according to an AoT source.
The source also denied Mr Makin was pressured to quit by the National Council for Peace and Order, insisting Mr Makin planned to step down long ago.
More changes at the top of the AoT could happen this year as about 10 executives are due to reach retirement age at the end of next month, the source said.
Among them are three AoT vice-presidents and five airport chiefs, including Suvarnabhumi airport director Raweewan Netarakavesana.
Don Mueang airport director Jaturongkhapol Sodmanee, who is scheduled to retire in two years, is expected to be given a new position, the source said without elaborating.