Why Indexing count going down ?

In 30 days time, we saw Gogole indexing more than a million pages from our website. But suddenly for the last 10 days we see there is slight drops in the number of URL in web index shown by Google webmaster tool. To be precise it indexed about 1.2 million pages in 30 days and in last 10 days it dropped about 80,000 pages. We are little concerned about the decreasing indexed count.
Any body have similar experience or would really appreciate if someone can help us understanding this.

Not much issue, it happens some times... Google picks new pages or sites quickly but later on stops showing... It will start releasing again...