As you know, I'm not a fan of this sport. It's too brutal for my liking. But I respect its participants.
I participate on a discussion forum with quite a few retired fighters, including a number of former world champions, some active fighters, and some top boxing writers. It's an informal, relaxed group, that has lately been taking most of my internet time. I was somewhat surprised that, going into this fight, a number of them had favored Holly Holm.
So, I asked my son what he thought. He said that a big factor was that although Rousey had been doing some boxing training, it appeared that she had prepared exclusively with right-handed opposition in the gym. Holm, of course, is a south-paw. Thus, everything from cutting off the ring, to avoiding and countering punches, would be "backwards." And that can often prove very difficult.
This is pure speculation upon my part: but, I assumed the thought was that Holm might be able to take the fight into deep waters, and pull out an upset late in the bout. That is, of course, largely what Buster did with Mike, though he was in control of much of that bout. I do not think that many people thought Holm would take Rousey apart this easily, much less crush her in the second round. But, looking at the entire bout, Holm was catching her, over and over again, with clean, hard punches. And Rousey was keeping her head a stationary target, with her chin up, and zero head movement.