It was a home invasion, quite literally. It was McCollough's home, and the fella invaded the privacy of that home without permission or invitation.
Now, neither you nor I nor anyone else here know what happened after the invader walked in uninvited, we only know the version he told the police. We don't know how far he entered, whether he acted aggressive, confused, or apologetic after being confronted, or how quickly he departed. We do know that he told the police at one point he mouthed off to McCollough, "don't be such a dick," but we don't know what was said by either party before or after that. That is a belligerent statement, I think you'll agree. So it wasn't all curtseying and begging forgiveness, apparently.
And Dude didn't just go to the wrong door. he went to the whole wrong building. He had to have been pretty distracted, or high/drunk, or both, to get the whole wrong part of the apartment complex.
BTW, here's what McCollough's building looks like courtesy of Google Street View and the unredacted address in the police blotter report:
View attachment 499850
We've all seen apartment complexes like this: breezeways with concrete landings and stairs for each set of apartments, 4 apartment entries on each landing, stairhead no more than 8' or 10' away from each of those four doors. So the dude saying McCollough pursued him 30' to the stairs is probably off by about 20'. There is some reasonable expectation of relative privacy up on the top landing, where a resident would reasonably only expect one of his three neighbors or their guests to be.
And if the dude fell down stairs after being hit, he only fell about 10 vertical feet, because these apartments typically have 8' or 9' ceilings. So about 10' between stories.
Is this significantly different than you imagined it? Because it seems significantly different than you were describing it.
Couple that with McCollough and perhaps his GF shouting in surprise and shock while this dude is mumbling some sort of perhaps-apology, and you have room for a whole lot of misunderstanding in a very short time period and fairly limited space.
I've never seen a doorbell camera or security camera in the breezeways of these types of apartments. One could be there, but I wouldn't count on it.