Board chairman Justin Milne said her relationship with the government could have been better, and she was not the right leader for the ABC.
"The board felt in the end that her leadership style was not the style that we needed going forward. We needed a different leadership style," Mr Milne told the ABC.
Ms Guthrie's contract contained a clause allowing the board to sack her without a reason at any time, which they did on Monday.
"I believe there is no justification for the board to trigger that termination clause. I am considering my legal options," she said.
But some ABC staff welcomed the move.
Senior journalist Sally Neighbour, executive producer of 4 Corners, tweeted: "Excellent decision".
Melbourne radio broadcaster Jon Faine said Ms Guthrie's term was an "astonishing fail".
The first woman to run the ABC and a former Google and News Corp executive, Ms Guthrie was appointed in May 2016 to drive digital transformation.
Mr Milne said her relationship with the government was a factor, but there had been no government pressure.
"That is possibly an area that could have been better ... That said, I think Michelle tried hard and was involved with government," he said.
"(But) the government, I must say, has provided no pressure, no opinion, has not been involved in this decision. This is entirely a board decision."
Mr Milne conceded Ms Guthrie had done a "pretty good" job turning the ABC towards its digital future.
"At no point have any issues been raised with me about the transformation being undertaken, the Investing in Audiences strategy and my effectiveness in delivering against that strategy," Ms Guthrie said.
David Anderson, a 30-year ABC veteran, will be acting managing director while a formal search for a replacement begins.
In an email to staff on Monday afternoon, Mr Anderson said he was honoured at the appointment.
"I want you to know that I am passionate about the ABC's purpose, public broadcasting and providing valuable services to Australians every day," he wrote.
"We all know that we face challenges around budget constraints, increased competition and changing audience behaviours. But we have a clear plan for the present and a vision for the future."
The Media, Entertainment and Arts Alliance said Ms Guthrie's replacement has to protect the ABC's independence and editorial decisions.
"This includes refocusing daily journalism away from lifestyle content and 'clickbait' and back towards news and current affairs," the union said.
Prime Minister Scott Morrison was told on Sunday night Ms Guthrie was going to be sacked, but said the ABC board made decisions independently.
Deputy Labor leader Tanya Plibersek said she was perplexed by the decision and the board's statements had not clarified it.