Hello Balakiran,
this behaviour is often caused by the so-called 'Postback settings'. By default this is set to 'Only when necessary for correct rendering of the form'. But in many times you have to set it to 'Always'.
You can change the default postback behavior for the drop down control control using the Postback settings options on the Browser forms tab in the Properties window for the control. There are three options available: Never, Always, and Only when necessary for correct rendering of the form (recommended). You can reduce the number of postbacks by changing the setting to Never. If you select Never, then the control will not send data to the server on change; the data will only go to the server and formatting, calculations, etc. will only run when the form user saves or submits the form, or another control in the form causes a postback to the server. For controls you want to change, you must change the postback setting option on each control individually --- and I personally recommand 'Always' in this case.
Regards,