Right around the time he was approaching the foggy valley crossing he had been switched to a different tower and had to tell them where he was and where he was heading .... right when he was intensely focused on what was in front of him .
Call it a minor distraction or whatever .... but our brains lose focus when we communicate by phone or radio while driving or flying.
That's when, in a well equipped twin, you use the upper mode functions of the AP to offload you - if you are too low and fast to do that then you have already made significant errors of judgement such that the HAI's Land and LIVE advice is the best solution.
Head down changing freq/squawk at low level is not clever - another reason that twin pilot Ops for this sort of work are better.