I think u used any script to send those emails to your yahoo account using same yahoo address as sender and your script was not smart enough to bypass the spam filter of yahoo. I faced this type of situation trying to send emails using PHP. Hotmail and yahoo recieved them as spams where gmail welcomed them to the inbox. So Yahoo is as smart as it was before
The spam filter is kinda of strict, a lot of things I send or receive end up in spams although they are certainly not spam. One way to deal with this is to change the spam setting or you will forever be in the bulk folder.
Could be two reasons why it dropped your email in to the spam folder 1. The content or the way the html of the email was structured scored some negative weight, enough to be considered spam. 2. If you sent the email to yourself from Outlook via POP and used a non Yahoo SMTP server that has bad marks against it then that could also score negative weight. Only the headers of the email will really tell you more. If however you sent the email from your online yahoo account to yourself and it got marked as spam, then thats stupid, you should really be whitelisted.