It is because an NSOperationQueue is not the main thread. What you're doing is illegal. And the sad thing is that there is no need for it! Just say:
[NSURLConnection sendAsynchronousRequest:urlRequest
queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
// ... and the rest exactly as you have it now
All fixed. Your request is asynchronous on a background thread, but when it comes back to you on the completion handler, you'll be on the main thread and able to talk to the interface etc.