Sometimes when using TOR browser / tor proxy, Google refuses to process the request you issue, especially if it's in a different language (e.g. searching for Arabic word while using German proxy).
For me I thought that its some kind of DoS attack prevention as Google might think that you're a bot who is trying to overload the engine with spam search requests.
But why the message error says that it wont process the query in order to protect their other users?
Is there some kind of queries that compromise other client on the internet using same search engine?