The price these cornerstone investors paid was unclear, since many of them were existing shareholders at the time of the IPO.
Adaro shares, which closed 2 percent lower on Thursday, have fallen around 14 percent from an IPO price of 1,100 rupiah.
UBS, which has a "buy" rating on Adaro, said in a recent report the cornerstone investors could make a huge profit from the share sale.
Analysts said investors may want to sell given the uncertainty over coal prices amid an economic slowdown.
"I think the slowdown in global economic activities made the coal outlook a bit sluggish over the next 12 months," said John Teja, head of equities sales at Ciptadana Securities in Jakarta.
"Moreover tight global liquidity might force these investors to cash in."
Adaro said shareholders had not briefed them about their plans.
"They are our existing shareholders, but we have not been briefed on the plan that they have with regards to their ownership," Andre Mamuaya, director at Adaro told Reuters
CONTROVERSIAL IPO
The IPO raised eyebrows among domestic investors and some global funds, who were shut out of the country's biggest public offering as shares were distributed to connected parties. [ID:nJAK22790].
Several analysts including UBS have written in recent weeks that a share selldown was imminent.
However, not every investor is looking to exit Adaro, as GIC and Kerry Coal, a firm linked to Malaysian billionaire Robert Kuok, will likely retain most of their stake, said a third source familiar with the deal.
Shares of Adaro overtook the country's biggest coal miner Bumi Resources (BUMI.JK: Quote, Profile, Research) as Bumi's parent faced financial difficulties last year.
Adaro, which expects to produce 42-45 million tonnes of coal this year, up from 38.5 million tonnes in 2008, plans to spend up to $100 million to acquire a coal barge and trans-shipment firm to improve the supply chain for its operations in Kalimantan on Borneo island.