User blog comment:LuNa/SBS volume 67/@comment-3337232-20120910035234/@comment-176.10.225.206-20120911055504

he freaked out like Usopp and Luffy did

I bet it was because Usopp was there. I mean, according to Oda, the reason why Luffy acted that way, when seeing Nami naked, was because Usopp is a bad influence on his behavior. I can imagine the same thing goes for Chopper.