The 16 Democrat MPs who voted to support Srettha Thavisin of the Pheu Thai Party as prime minister on Tuesday had defied the party resolution to abstain, caretaker Democrat Party leader Jurin Laksanawisit said on Wednesday.
They had been asked to explain their actions, and could face in internal party investigation, he said.
Sources said the 16 renegades had been hoping for an invitation to join the new government coalition.
Mr Jurin said a Democrat Party meeting on Aug 21, ahead of the Aug 22 parliamentary vote, passed a resolotion that its MPs should abstain, with the exception of former party leaders Chuan Leekpai and Banyat Bantadtan, who asked for and received party permission to vote against Mr Srettha's nomination.
In defiance of the party resolution, 16 of the party's 25 MPs voted for Mr Srettha without giving prior notice.
Mr Jurin said he had ordered the renegade MPs to explain their actions at the next party meeting.
If party members called for the party to investigate the 16 MPs he would set up a committee to investigate them, although MPs did have the individual right to vote as they wish.
Mr Jurin did not know whether the 16 would be punished, saying only that penalties were set down in the party regulations.
Asked about a suggestion the 16 MPs had defied the resolution so that they would be expelled and could join a party in the coalition government, Mr Jurin said he had no idea. Only the MPs themselves could explain why they chose to do so.
The caretaker Democrat leader insisted the party had never assigned anyone to talk with other parties about joining the new government.
Sathit Pitutecha, a caretaker deputy party leader, said the 16 MPs had committed serious misconduct, but whether they would be expelled rested with an investigation panel. Since the misconduct was serious, the party leader could set up an investigation, he said.
Mr Chuan, a party list MP and former party leader, said it was "unbelievable" that the 16 MPs had voted in defiance of the party resolution.
He had asked for and received party permission to vote against Mr Srettha's nomination because the Democrat Party had been at odds with the Pheu Thai Party over the distribution of development budget funds to the South, the Demcrat Party's traditional support base. He accused Pheu Thai of curtailing funding to the South, saying it was an act of persecution.
After Tuesday's vote for prime minister, 15 of the 16 renegade Democrat MPs made themslves unavailable for comment. The exception was Pol Maj Gen Surin Palare, a Songkhla MP. He said only that he voted for Mr Srettha because he wanted the country to move forward.
Sources said the 16 MPs belong to a faction led by Chalermchai Sri-on, the caretaker party secretary-general.
Before the vote on Tuesday, their leaders - Det-it Khaothong, a Songkhla MP, and Chaichana Detdecho, a Nakhon Si Thammarat MP - talked to Phumtham Wechayachai, a Pheu Thai deputy leader. They told Mr Phumtham they were ready to vote for Mr Srettha if he looked like falling short of the needed 375 votes, in return for being included in the coalition.
While the voting was going on in the parliament chamber, the 16 sat outside, waiting.
When the support for Mr Srettha exceded 375, they told Mr Phumtham that they would vote for Mr Srettha anyway. They allegedly offered to fill in in the event the Bhumjaithai Party, with 71 MPs, withdrew after failing to get a premium grade ministry.
The 16 Democrats then entered the parliament chamber and voted for Mr Srettha, the sources said.