Why the switch? The obvious answer would be so that there could be a roller coaster.
One way to look at it is that sometimes the descent off a mountain cannot be the same as the ascent. I've done a few hikes where I look back at what I've climbed and thought, why did I do that? This side is easier.
The Yeti animatronic is the most important plot device in the Everest storyline. He's the "big reveal" at the end, the glue that holds the whole ride together. The entire experience builds up to that quintessential moment when the riders encounter the ferociously lunging Yeti, but because he is immovable, hidden, and a shell of his former glory, the narrative resolution of the story is completely ruined.
I am saddened by the fact that my beloved Disney still has not fixed this attraction, and further saddened to hear rumors that the fix is going to be a CGI solution that will make the Yeti appear to be moving. Pathetic.
I am just glad I was able to ride EE back in 2007 when the Yeti still worked. It used to be my favorite Disney experience, now it is not even my favorite attraction at Animal Kingdom.
As for the backwards portions fitting into the storyline, I like M. Ryan's reasoning about needing to find a different path on the descent...but also the fact that your escaping near death will make you shoot down that mountain any which way possible, even if it means scrambling down on your backside!
BTW - which part of the train feels the fastest? I've tried front, back and middle, but had a hard time making up my mind...
As for the Yeti, had you ridden when he actually moved, you would know the difference, trust me. He was so prominently displayed back in those days (he's almost completely hidden now), that your time with him seemed immeasurably long.
The lunging Yeti is to Everest's storyline what frosting is to cake: one is insufficient without the other. You simply cannot produce an astonishing exclamation without the use of an exclamation point! The Yeti's final lunge as your car passed just out of his reach was Everest's exclamation point.
The current version of Expedition Everest, while still a fun ride, is a vastly diminished experience without a fully operable, lunging Yeti.
I refuse to let Disney off the hook for the current state of the Yeti. Once you have experienced Expedition Everest with an A-Mode lunging Yeti, all other modes of operation (B-Mode, Show-Mode, Crap-mode, and/or Dance Club Mode) just aren't good enough. Not for me, and definitely not for a company with the clout, money, legacy, and history of the Walt Disney Corp.
Disney Magic should never fall into this type of disrepair, and it should never be allowed to become second rate.