Brook was doing great until his eye ****ed up

I'm rewatching the fight. Round 1 and 2 were close. Round 1 to Golovkin because he had Brook hurt, but Brook closed the round pretty well. Close round for Golovkin. Round 2 for Brook, he made Golovkin look like a C level fighter in some periods. Then comes Round 3, and when it starts Brook touches his eye, which shows that at the beginning of that round he already had the ****ed up eye. Starts the round uncomfortable, a shadow of what he was in Round 1 and 2, and starts getting beaten up. I'm watching the fight from here http://www.boxingfightsvideos.com/2016/09/gennady-golovkin-35-0-0-vs-kell-brook.html I still don't understand that second round. In the first round I thought... well Brook is fast, which was understandable. In the second round Golovkin just looked like a complete tool. Still he managed to get Brook's eye broken, luckily for him. I don't know if this is the result of Vada testing or what, but he didn't impress me as much as in his other fights. Also his defensive skills were horrible in this fight, and I've seen having much better defense in other fights.