I resolved the issue by going through a couple of more blogs over net.
What I was doing was I wrote these entire set of controls in asp Panel.
The viewState property of this panel was set to False.
As soon as I changed it to true, the unwanted selectedindexchanged event got suppressed.
This got me through.
But, it was a hunch.
I did not yet got the reason why this happened ?
Explanations are most welcome.
@Anil: Thanks fot the concern.