Marciano was whipping that left hand the whole second round; he missed badly with a few, some were blocked, but he landed a few. Watching it again, there seems to be a spot where a left hand has an impact on Mathews- by the 4:00 mark he seems shaky. He fought such a beautiful first round and then a few seconds.
I don't think he did anything 'wrong'. Sometimes when you are thinking, the other guy is thinking too. Charley Goldman was very astute and he had a minute between rounds to point out that, by avoiding the right, Mathews made himself vulnerable to the left. But Mathews fought a nice first round; that was the way to beat Marciano.
Of course, it would end up being trial by fire, but that is what Moore and Charles and Walcott saw. Make him reach then fight in with him. I haven't seen the Moore/Marciano fight in a long time, but I'd bet that the knockdown moore scored is similar to what Mathews was trying. Get Marciano to wing the right from too far away and counter with your right.