In case you are using the Standard Twitter button please keep in mind that its original implementation from twitter has issues with querystrings, so this may be causing the problem in your case. Could you try using the HttpUtility.UrlEncode() method before setting the URL for the twitter button and see if this helps?
tb.UrlToShare = HttpUtility.UrlEncode(url);
//tb is a reference to the TwitterButton
This will confuse their engine and it will miss the query string.
Another alternative is to use the styled twitter button instead:
the Telerik team
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