Ok... Now I have another strange observation:

1. Search done. Result populated in gridview
2. Clicked Select commandbutton in Gridview row. Internally generates error "Object reference not set" against .SelectedDataKey property
3. Now again Clicked Select commandbutton in Gridview row. This time, the data is populated in "B"
4. Any further search and select works well.
5. It's the first time select that it nags. Seems weird but I guess there's something to do with page rendering OR gridview databinding OR updatepanel. 

I'm even more confused now.