Ok so I ran Grab! again this morning and can expand on that blurb about it from earlier, now.
What happened was I was doing my morning caffeine over a Grab! and the first thing that happens when we walk in the door is that the cutscene starts. So, half awake, I assume I landed in a Brazilian death squad team and did my thing to blend in - I ran up the stairs and back to the balcony where all the nameds are.
I start wailing away, trying to gather up a crowd, doing my thing, half awake, and then I start to realize that no one is there but me and no one seems to be coming, either. Next thing I know I'm de-virginating my new Nimbus and Supernatural and popping anything else to keep alive.
No one ever showed up, and after the big balcony fight there were still a number of mobs left to take care of before we could exit, so I went and joined the rest to finish up.
Afterwards, I couldn't remember if the cutscene was supposed to be at the first of Grab! or further in. I can't remember seeing any names when I was fighting, either, due to it was kinda intense and I was kinda not awake.
That's why I said I "think" I soloed all the Grab! bosses by accident.
Just to clear that up..
Somewhat.
I still dunno why the cutscene triggered on entrance.
