I don't know the exact differences between the various seop factors that affect rank in Google versus the factors that affect rank in Yahoo. However form my own experience of being actively involved with marketing my websites online, I have found that effects are first seen in Google. Maybe it could be because we mainly focus our
seo campaign and strategy basedon Google. In due course howevere Yahpp will start to pick up and then you will see a steady increase in traffic form Yahoo. Maybe this patters only applies to my sites. I could be a coincidence but I think this is likely to be the case with your website as well.