Apparently, when the impact hypothesis was first presented as more than just speculation (i.e. with the iridium layer as evidence), a majority of paleontologists rejected it, preferring one based on long-term high-intensity volcanism in Asia. I believe this hypothesis still has a few proponents.

The collision of comet Shoemaker-Levy with Jupiter occurred as several distinct impacts, due to gravitational disruption of the comet in a previous close encounter with the planet. Something similar is clearly possible on Earth (perhaps even gravitational dispersion of a comet passing close to the Moon just before impact?) but the Chicxulub crater is large enough to explain the extinction event by itself (unless you hew to the volcanic explanation, I suppose), so the question is whether there is any evidence for multiple impacts. As far as I know, the global distribution of the physical evidence is indicative of a single source, but not definitively so.

At that point, they didn't even believe the volcanism hypothesis, the bias toward gradualism was so profound.

It’s a good theory, and may have occurred by large objects breaking up during atmospheric entry or it was a cluster of objects in space.

The K/T impactor had linear dimensions comparable to the scale height of the atmosphere. Unless the impact was at an extremely shallow angle (unlikely) breaking up in the atmosphere would have no significant effect. The mass of atmosphere that could potentially transfer momentum with the impactor before impact would be much less than the mass of the impactor.

Wouldn't the break up be more influenced by the gravitational acceleration imparted by earth? Most asteroids are rather loosely bound.

That would be tidal forces. There wouldn't be enough time for that.

Shoemaker-Levy orbited Jupiter for quite a while before it broke up and went in.