the update panel replaces all the html in the panel with what is sent back
from the server. The browser makes an attempt to compare and reduce the
flicker, but is not always sucessful.

I would not use an update panel for something as trival as reloading a
dropdown. use a simple ajax callback. you shoudl also disable the dropdown
while waiting for the async return. to see why, in the dropdown hit the
up/down arrows, you site will probably error out.

-- bruce (sqlwork.com)