Let's not forget that Kaido is basically just playing around all this time. I mean, he was constantly spamming the same attack over and over again, but he knew it wouldn't do anything.
I think, after he got hurt again in the same place where Oden damaged him, he had enough of it. And, he then decided to really get rid of the scabbards.
Also, I guess he just wanted to test them too and see how strong they actually became. Now he will fight back the proper way, just like he did against Luffy. There is no need for him to hold back.