James Johnson, 83, of Ffordd Elfed, Wrexham, a stalwart of the local golf scene, died in January last year at Wrexham Maelor Hospital.
Mr Johnson had been a patient during one of the major waves of Covid.
Respiratory consultant Dr William Jones told the Ruthin inquest he had never come across a tube being disconnected before and that Mr Johnson's fatal decline would have occurred in a "small number of minutes".
A post-mortem examination showed that Mr Johnson had Covid pneumonia.
Dr Kath Clarke, assistant director for patient safety who chaired a serious incident review for the health board, said the tube becoming disconnected from the mask was "really, really unusual and still is".
No fault was found with the continuous positive airway pressure (CPAP) machine, she said. It was possible Mr Johnson may have leaned on the tubing but the investigation into the tragedy was unable to decide what happened.
A nurse and a healthcare support worker, wearing personal protective equipment, were looking after eight patients in two rooms. Ideally there should have been two patients to one nurse, the inquest was told.
Nurse Catherine Norgrove said patients were observed through windows and the "obs" machine was turned so the screen could be seen. But when no-one had been in the rooms, machine alarms could not be heard.
At 1.30pm she looked at Mr Johnson's monitor. "I felt there was something not right," the nurse said. A doctor then told her the tubing wasn't attached.
Coroner John Gittins recorded a narrative conclusion. He said the widower had been admitted to the hospital on January 3 last year and treated for Covid. A week later his declining condition required CPAP respiratory support.
Due to the pressures of the pandemic it was given using a machine normally utilised for home care.
The coroner said: "On January 13, staffing pressures meant that only two members of staff had been fit tested, allowing them access to the room where Mr Johnson was being cared for. Together they had to care for eight patients, whereas optimum care in a non-pandemic scenario for CPAP patients would have been one nurse to two patients."
It was then noticed that the pensioner's oxygen saturations had dropped and after his death it was realised that an oxygen tube which fed into the CPAP mask was no longer connected.
The coroner remarked :"It's not possible to establish how the oxygen tube had become disengaged. However, the reduction in the delivery of oxygen to Mr Johnson would have led to increasing hypoxia and it is probable that this would have hastened his death."
The coroner said it was a rare situation at a time when the NHS was under "severe" pressure.