Taking into account the info we get about this incident in chapters 49/50 what I gather is:
Miyamoto was dating Fujino. Long put a note in Fujino's friends' shoebox saying it was fine to 'share' Miyamoto. Page 22, chapter 49 it mentions 'You like more than one' it said. Which was most likely on the note Long put in the shoebox. So that group of guys assaulted Miyamoto. Long's plan in the meantime was to be the Miyamoto's savior. While he set up the assault he would swoop in to protect her and get her to look his way. But before he could 'save' her she fought and ran (as said on page 19, chapter 50).
Meanwhile Long has always regretted what he has done and tried to protect Miyamoto from the shadows. As seen on page 10, chapter 50 Long is the one wanting to hide it the most. To pretend it didn't happen. It's just time and time again he makes himself look like a worse and worse villain. I hope this cleared it up a bit!