Experts Please Help: WordPress Bad for SEO?

Hi,

I got a case on WordPress for SEO... a really weird case.

Say I planned on targetting 5 long-tail keywords:
"keyword 1", "keyword 2", "keyword 3", "keyword 4", "keyword 5"
My domain name for the WordPress blog is: keyword1.com
and the install was at the domain root: keyword1.com

I put my target keywords two times in the posts, in bold and italics.
I had All-in-One SEO Pack installed and used properly when I posted.
Also there's Smart Update Pinger so I can get my blog post indexed
by Google in a matter of hours. I' wrote 3 articles for directories in
2 weeks period, with the anchor text and link pointing to blog domain root.

In the first few weeks, it was doing OK.
"keyword 1" blog post and blog domain root climbing in the SERPs,
also "keyword 2" to "keyword 5" all climbing to compete to rank on
their own keyword phrase. Eventually, my blog domain root
keyword1.com ranks #1 in Google for "keyword 1" and stayed for days.
My other keywords were also doing well: ranked 3 - 17 in Google.

Then something happened...

Not only all my ranks dropped (which is common, as far as I
understand it) but my blog post "keyword 3" is replacing my
already ranked "keyword 1" post and blog domain root SERP.
Now out of all the keywords I'm targetting:
- 3 nowhere to be found in the first 50 positions
- 2 in top 10 for their own respective keyword "keyword 2" and "keyword 3"
- when I search for "keyword 1", instead of my domain name or
my "keyword 1" post... the one that ranked were "keyword 3" post (#15)

How can things like this happen?
How can another keyword post intercepts and outrank the one I actually SEO-ing?
What did I do wrong or less effective in SEO-ing a WP blog post?



Thanks in advanced,

Ray