Do you mean that your Alexa rank i.e. the number is going down (which is a good thing) or the number is going up?
I really do not know why you all concern this much about Alexa, PR, and so on.....they do not bring you any income...
According to me there are many reason to drop down your alexa rank of your website.Alexa rank is depend on your site's traffic and visitor.How many traffic to your website have and how many visitor are visit your site's.So update your site regularly and add some new content in your website, your site content is fresh and some informative information means definitely visitor are visit your site and also traffic will increase.
don't worry about alexa at all, create useful content, promote your website, use Google analytics, webmasters tools, and you will be good. alexa is only for top 100 000. don't relay on alexa.
Alexa shows the traffic rank of your website. It generally happens. Day by day, more & more new & good sites are getting listed in search engines. So, the rank can differ. But you should concentrate on the contents of your site. Try to increase the visitors base of your site. Also, proper knowledge of a site can be get from google analytics only.