This is a suggestion, I'm not sure this was the end of the story, but...
In 2008, a problem of arcing near the Pyro bolts was proposed as an explanation by the Russians.
A Pyro bolt was later retrieved from a docked Soyuz (TMA-12 -- Also here), and returned to earth for examination.
TMA-12 undocked with the remaining Pyro bolts and returned normally on Earth. The returned Pyro bolt examination was not conclusive, but other data existed.
According to Nasa Spaceflight Russians engineers stated:
Following the investigation, the Commission found that the most likely
cause of the failure to open the latch located at “Plane I” of the
interface, “was a failure of the 8 X 55 explosive bolt, caused by
combined long-term exposure, under space flight conditions, to
electrical discharges resulting from the difference in potential
between the station hull and the surrounding plasma, which increased
in magnitude after the electrical power capacity of the ISS was
The findings relating to the long-term exposure to electromagnetic
emissions on-orbit, and their potential to cause issues with the pyro
bolts, came after an extensive investigation that included the removal
and return of one pyro bolt from Soyuz TMA-12.
NASA confirmed somehow the assessment:
Our independent analysis agreed well with Russian provided data.
Pyrotechnic and environments experts believe the stated most probable
cause is possible, however unlikely in our experience.