Kelly Crow is first out with an explanation in the Wall Street Journal:

Dealers say the surge in casts of Giacometti’s works made after his death in 1966 also has made works crafted during the artist’s lifetime like “Walking Man I” more valuable. New York art dealer Marc Glimcher said the price for “Walking Man I” also might simply reflect the tenacity of the final two bidders: “Above $50 million, the fight for any artwork goes from love to a grudge match.”

Giacometti is a new favorite of Russian collectors like billionaire business magnate Roman Abramovich, but a person familiar with the matter said that Mr. Abramovich wasn’t the buyer of “Walking Man I.” Mr. Abramovich caused a stir at the Swiss art fair Art Basel two summers ago when he bought one of the artist’s 1956 bronze figures of a woman from New York gallery Jan Krugier. That work had an asking price of around $14 million.

The Master, Judd Tully, gets this out of the specialist who bid for the buyer about one of the underbidders:

Sotheby’s senior specialist Philip Hook, who took the winning phone bid at a hammer price of £58 million, said that one of the unidentified underbidders told him before the sale that he had been waiting 40 years for the sculpture to come on the market. It turned out to be that kind of generational event. Hook declined to divulge any information about his phone client.

The Guardian gives these details about the bidding:

There was a genuine sense of anticipation in the auction room. Not only could you smell the expensive perfumes and colognes, you could smell the money. Interest in the sculpture was clear from the start with bids being shouted before the auctioneer had even had chance to ask for them. “On your marks, get set, I’m going to start at £9m,” said auctioneer Henry Wyndham. “£12m,” came the first bid. “That’s my kind of price,” said Wyndham. The figure then rattled up quickly, ping-ponging around the room. In total there were 10 bidders but it came down to two telephone bidders from the mid-£30m mark onwards. When it went from £47m to £50m in a giant leap – what’s £3m after all – there were gasps. When the hammer went down, there was loud applause.

Colin Gleadell of the Telegraph adds this pricing benchmark from before the sale:

The American art dealer, Larry Gagosian, who handles the Giacometti estate, was known to have another cast of the same sculpture priced at 45 million dollars.

Sotheby’s Sells Giacometti for Record $104.3m (Wall Street Journal)

Giacometti Work Becomes Most Expensive Work to Sell at Auction (ArtInfo)

Alberto Giacometti Breaks Auction Record with £65m Sale (Guardian)

Record Alberto Giacometti Sculpture: An Appreciation (Telegraph)