my site is doing great in google, but in the last 3-4 weeks dived in yahoo. I used to 2-3 for some keywords, am now sometimes not within the first 10 pages. What happened? I emailed them, and they gave me BS about how yahoo updates its index BS. It is an established authority PR4 site. I have no clue
I lost 16 positions for my main keyword in yahoo, after two years I was in top five. They shuffled out a new update that sux big time. Wiki is all over the place as well as results from social media sites. A post of mine in yahoo answers rank better then my site on the same topic... go figure.
I don`t think you can ever really understand how search engines work out which keywords for what sites will show up well. If you are doing well on Google I would suggest you don`t worry too much about the others.
It's difficult to determine what has caused this change, but this can happen when yahoo changes their algorithm.