First attempts where in vain to deactivate viewStates from the control or change webpart-attributes. A more promising approach seemed to be deleting browser-cookies. Reloading Firefox after deleting cookies worked like a charm but I've got no success to reproduce this behaviour by code.
So I had to try another way and began to debug the SharePoint-JavaScript by using Firebug. That was a good decision because I discovered this method:
ProcessDefaultOnLoad(_spBodyOnLoadFunctionNames);
In here a simple array is iterated over that's items are getting executed by eval-function.
One of the array-items is "ExpGroupOnPageLoad". This function cares for expanding each group if it's found in the state-cookie.
A little bit googling about the JS-Variable _spBodyOnLoadFunctionNames gave me a little bit more overview and so I tried to remove the ExpGroupOnPageLoad-entry in hope that the groups stayed collapsed after reload. And voila! It worked like a charm!
Just add this piece of code in your AllItems.aspx (or another view-file) and the list-groupings stay closed after page-reload, no matter, what groups the user expanded before:
<script type="text/javascript">
for (var i = 0; i < _spBodyOnLoadFunctionNames.length; i++) {
if (_spBodyOnLoadFunctionNames[i] == "ExpGroupOnPageLoad") {
// remove only this one element from array
_spBodyOnLoadFunctionNames.splice(i,1);
break;
}
}
</script>