For some reason, the SSRS 2005 report that I’ve deployed on MOSS 2007 perform auto postback whenever a particular drop down parameter value is changed. It’s just a normal text drop down parameter with a default value. Some online post says not to use the “=SomeText” syntax, but mine is just the text directly set as the default value.
Luckily my colleague gave me the tip that the sequence of parameters might cause this behavior. Although there are no dependency between all my parameters, he advised me to put parameters that are dynamic at the top first, follow by the static ones at the bottom. That solved my problem!