It is clear the pilot wanted to be at 6500 - the only plausible reason for the descent is that he believed that was an instruction from ATC. Otherwise he would have continued tracking around the western side OCTA at 6500 rather than turning back on track and descending.
If he knew the terrain was there presumably he wouldn't have flown into it. It seems likely that, believing he had an instruction from ATC to descend on track, he assumed that terrain clearance existed to do that.
It does raise an interesting point:
If ATC issue a clearance at a level different to your current level, without a specific instruction to enter CTA at that level, do they expect you to continue on track and descend to the cleared level, or are you expected to circle or whatever it takes until you can enter at the cleared level?