from SCMP article,
"Some military experts believe the vessel was forced to surface, but others say there is not enough information to back up that theory."
“This is such a shame for the navy,” said a Beijing-based military source, who requested anonymity, adding that the vessel was detected because it was “too noisy”.
"It is also unusual that a nuclear submarine – which could stay underwater for months – surfaced in front of another navy, given that they usually strive to stay unseen and undetected."
“Once a submarine has been exposed and its unique acoustics have been recorded, it puts them at a great disadvantage,” said Li Jie, a researcher at the Naval Military Studies Research Institute in Beijing.
Macau-based military expert Antony Wong Dong believed the nuclear attack submarine was forced to surface, and said it was “dumb” of the Chinese navy to allow its features to be seen and photographed.
He also dismissed claims that the submarine was flying a Chinese flag to assert its claim to sovereignty over the Diaoyus, noting that it surfaced in international waters.
“If they wanted to claim sovereignty, why didn’t the submarine enter the territorial seas of the islands?” Wong said.