He's been acting imperious and aloof for awhile, dating back to around the time Dr. Octopus died. He became way more aggressive. The Avengers noticed this change themselves and entertained the idea that he might be a skrull, so the idea that someone is impersonating him is out there. And finally, he's been using the extra four leg harness that looks exactly like the one Doc. Ock used. It's all there for someone to put together.