Based on the 'facts' available through the media, a decompression event is the most plausible scenario. Whether or not this was a slow (10000' cabin alt warnings??) or rapid decompression is unclear.
Are YOU a pilot? At FL350 you wouldn't be instantly blue in the face and pass out... Do you KNOW how long it takes to don a quick-donning oxygen mask? It takes me two (2) seconds to pull the mask out of the slot with one swoop of my left hand and plant it on my face.