When Luffy tries to head to the Rooftop to face Kaido, Zoro declares that he would go with him:
Normally, Zoro would stay behind to defeat whatever subordinate of the arc antagonist is blocking Luffy's path, but not against Kaido. Zoro breaks from tradition and declares that he'll accompany Luffy to the battle against Kaido.
While they are rocketing upwards, Zoro questions Luffy to verify that Kaido was indeed on the Rooftop. Luffy affirms that he is, and that Kinemon and the other Scabbards are also fighting him:
Again, Zoro is maniacally focused on getting to Kaido.
After Queen intercept Zoro and Luffy's trip and King blocks off the Rooftop, we see the both of them complain that they can't get to the Rooftop and that the Calamities have completely blocked off the sky:
Again — and I recognise that I belabour this point — but in traditional arcs, this would be when Zoro volunteers to stay back and hold off the calamities so that Luffy could progress and face Kaido. This did not happen (the role of staying behind and clearing the way to the arc antagonist was instead given to Marco).
Later on, we see the Strawhat crew rally together:
Luffy declares that he would take on Kaido and the others should handle the rest and watch his back:
The other Strawhats affirm Luffy's order and declare that they'll handle things:
Zoro reaffirms that he just needs to get to the Rooftop and confront Kaido:
Even in a scene where Luffy declares that he shall face Kaido and asks the Strawhats to watch his back, Zoro is singled out to reiterate his intention to fight Kaido. I don't know if Luffy was implicitly excluding Zoro from the declaration to handle the rest and watch his back, but if he was, then Zoro declared his defiance to a direct order from Luffy.
Whichever the case, Zoro was singled out from the other Strawhats that were to watch Luffy's back and handle the situation on the Live Floor.
Again, this is a massive deviation from the normal formula. This is another opportunity that Oda was given to have Zoro confront one of the calamities. Luffy literally ordered the Strawhat crew to defeat the enemies on the Live Floor, but Oda singled out Zoro and had him reiterate his desire to face Kaido.