He was surprised when Logan came back at him so hard and so fast. That would have slowed most people down, if not taken them out of the fight entirely. Logan, however, seemed far from slowed down. If anything he was more pissed off, and the feral in him rose to the challenge.
Somewhere in the middle of the scuffle Nik had slipped forms again, perhaps more comfortable as that giant wolf than he was in his bipedal form. Yes, it was a bit less suited to full blown combat, but it was how he was used to fighting. Fast and agile, trying to outmaneuver his enemy and hamstring him. Not that such a thing helped with Logan. The older man was everywhere at once, it seemed, and every blow landed with nearly bonecracking force.
The wolf was fighting back, at least. A bite here, a slash of claws there, but none of it was doing any significant damage that he could see. Even if he could smell some blood, his opponent seemed unhurt. The blow to his nose drew a howl of pain which quickly morphed into an enraged snarl as he turned with far more quickness than something that size should possess and snapped his jaws closed on Wolverine's hand and arm. The result was undoubtedly bloody, as flesh and tendon shredded, but ultimately futile as Nik's teeth met something they could not break, forcing him to release the man and try to leap away again, obviously favoring one leg now. Could be the other reason he'd shifted to all fours.