During a caucus meeting at Queen's Park on Monday night, he said that shutting down the legislature would allow his party time to choose a new leader and come up with "the next set of Liberal ideas," presumably to boost the party's chances of holding on to power in the province.
"This is the right time," McGuinty told reporters, saying the Liberals were in need of "renewal."
Behind the scenes, however, a number of factors may have forced him to step aside a year after winning his third consecutive election.
Did controversy force his hand?
McGuinty had been facing a growing scandal over the province's Ornge air ambulance service, which is under a criminal probe. Adding to his troubles was a rare contempt motion over the costs of cancelling two gas plants.
Proroguing the legislature shut down the government committee looking into the Ornge air ambulance affair. It also means the gas-plant contempt motion died on the books.
Conservative Leader Tim Hudak has vowed to re-introduce the contempt motion in the next legislative session, whenever it gets underway. But McGuinty's announcement has bought his party time to deal with both scandals.
In the words of Richard Mahoney, former president of the Ontario Liberal Party: "It will cool down the temperature a little bit a Queen's Park."
Were labour relations his downfall?
Many observers point to fraying relations between the provincial government and public-sector unions.
McGuinty had alienated a powerful ally — Ontario's teachers — by forcing a pay freeze to reduce the province's massive $14.4-billion deficit. The unions in turn declared war, vowing to withdraw their financial support and use their organizational might to defeat his party in the next election.
By proroguing, Mcguinty has "pushed the pause button" on labour talks, said Armine Yalnizyan, a senior economist with the Canadian Centre for Policy Alternatives.
"It creates a space for the Liberals to go out there and try and figure out what they're strategy is going to be," she told CBC's Metro Morning.
McGuinty's resignation could also prompt some movement in the labour talks, as unions leaders weigh whether to strike a deal with the Liberal premier on his way out, or face the prospect of dealing with a Conservative government after the next election.
Did family life play a role?
Monday's news conference was clearly emotional for McGuinty, who insisted it wasn't the controversy over the gas plants that prompted his sudden resignation, but rather cited his daughter's recent wedding as a time when he re-evaluated what was important.
"I thought it wasn't going to be that big of a deal, but I found it to be pretty emotional to be there with her and my family and my extended family, and it reinforced for me what those things in life are in terms of the most important: family, friends."
It's a common refrain for politicians who have decided to move on to other pursuits – including those who have become embroiled in scandal.
But in McGuinty's case, there were rumours his wife, Terri, wanted out of political life. She was invited to Monday's caucus meeting, which caught some MPPs by surprise, and sat in the front row as her husband made the announcement.
Was leadership style the problem?
Some experts say McGuinty was ill-suited to governing with a minority of seats in the legislature after last year's election.
His government "had a majority for eight years and was able to do what it wanted," said Henry Jacek, a political science professor at McMaster University in Hamilton, Ont.
But when it came time to "reconcile themselves to the fact they did not have a majority, that they were going to have to be far more flexible than they were used to," they ran into trouble.
That was particularly true in the government's dealings with public-sector unions, Jacek said. It would have made sense for McGuinty to reach out to the NDP for support on his public finances plan since the party isn't the official opposition, but McGuinty "is actually quite conservative" on economic policy and his proposal failed to win the confidence of New Democrats, he said.
Is he bound for Parliament Hill?
The premier said he would also stay on as the MPP for Ottawa-South until the next election, but did not rule out taking a run for the leadership of the federal Liberals.
"I am not making any plans whatsoever beyond my duties here at Queen's Park," McGuinty told reporters.
But for the past month, a draft campaign has been in the works to persuade McGuinty to jump into the federal fray. Some of his closest campaign advisers have reportedly been involved including Don Guy, campaign director for each of McGuinty's three winning provincial campaigns.
Mahoney, the former president of the provincial Liberal Party, told CBC's The Current that he is convinced Mcguinty genuinely intends to "have an orderly succession, allow the Liberal Party to choose a new leader and then fade off to a slightly more relaxed life."
However, he added that Mcguinty will be "under enormous pressure, to run for the federal Liberal leadership. There are many who have gone to him in the last number of months and asked him to do that."