Google has always been warning against automated position checkers which uses a lot of its resources.
Now Google has taken a stronger hand and has blocked Webposition Gold software from performing automated ranking requests in Google. AUtomate rank requests creates a lot of junk queries and uses a lot of server resources of Google. Google has been issuing warning not to use webposition Gold but people continue to use it. Now Google has taken action and has blocked all web position gold queries. Web position Gold has an unique way of sending queries to Google and it seems google was able to detect it using their bot filter software.
We at Search engine Genie never use bulk keyword rank checkers. Our rank checkers are search engine friendly and allows only limited queries per day.