I think the hypothesis of an explosive decompression in witch the pilots did the first steps and somehow didn't put the masks or the masks didn't work is the more plausible.
The first rough procedures in case of emergency descend, after putting the masks on is:
- ALT turn (descend) and PULL
- HDG turn and PULL
- SPD PULL
(followed by speedbreaks and refinement of the knobs)
It would justify the steady descend and speed. But why didn't the heading change? And while the PF did this, the PNF should be setting 7700 and calling Mayday. Something more must've happened...