Oh yeah, I remember when I first time had that "problem".

Wordpress tags are automatically generated from your content. And by default the tags can be searched via yoursite.com/tags/keyword. And for each keyword that is being generated, a tag page is being generated as well. So goes on and on and on, you might gain 20-30 tag pages for each article/page you create.

I would call it a real problem though. The more sites are index, the better for you. And if you fall in ranking, it's most likely that your competition is not sleeping. Create more backlinks.