This because the impactor arrived right around the same time in which the Deccan Traps (gigantic lava producing volcanic zones, like slow moving, continent-destroying lava flows basically) were apparently already in full swing, creating their own climatic havoc around the world.

Even now, enough evidence exists in favor of the Traps being at least a major cause of devastation at the time that it's hard to be sure.

Basically, it could be that the Traps were doing their thing and would have caused some high amount of species extinction all by themselves, but perhaps not as much extinction as did happen, or it could be that they could have caused a mass extinction by themselves but that the asteroid -even if the Traps had not been active- would have anyhow caused the mass extinction that happened at the time. Thus, though it's definitely known that the asteroid hit and caused global cataclysm, it's debated if this by itself would have been enough in the absence of the Deccan Traps.

Obviously, because the traps were already destroying ecosystems and species doesn't mean that the asteroid couldn't have caused mass extinction all by itself, and the speed at which species died off according to fossil strata does indicate the much more sudden asteroid effects as the main culprit, but the coincidence of both together does muddy the waters a bit, so to speak.