In the early days of Greg's YouTube fame, he offered all of his music for free on his website. This included his comedy songs, background music he created, and instrumental versions of his songs for other YouTubers to make parodies. He stated anyone can use his music in his videos, as long as they gave him credit.
At some point, YouTuber OutBackZack created a parody of Greg's "I'm A Banana" song. The video was called "I'm a MEAT EATER" (video now removed) and it featured Zack singing about eating meat in a hot dog costume, parodying the original banana video. (Source) In addition to being a parody, the video also seemed to poke fun at Greg's vegetarianism, which he was known at the time for making incredibly harsh and over the top videos to convince viewers to go vegetarian.
After attending VidCon 2011, Greg uploaded "Hater at Vidcon". In the video, Greg talks about a "hater" that approached him during VidCon. (This video was removed by Greg, so a summery can't be made.)
On August 6, 2011, Zack uploaded "RE: Hater at Vidcon" to his side channel in reply to Greg's video. He explains that he was drunk at VidCon and decided to talk to Greg. He says they shook hands and Greg did not recognize him at first. He says when Greg realized who he was, he immediately shut him out.
Zack says he was trying to be nice and polite. He told Greg, "Hey, this is VidCon. We're all here cuz we're content creators and I respect you as a content creator. Your energy, I find you funny a lot of times, and I love your editing skills. I respect you because of that. Yeah, we have different morals and values, your ying and I'm yang, but hey let's face it, we- You know, a philosophy of mine is that I exist because you exist and vise versa. I want to respect you at least for out common interests." He says he was drunk, so it most likely was not worded exactly like that, but that was the point he got across.
He says he remembers what Greg did and that other people confirmed this is what Greg did. Zack closes his eyes, "You're a scumbag. You stepped over my morals, you humiliated me in front of everybody. You're a scumbag."
Zack says he sarcastically replied, "I'm not a scumbag, I'm a douche bag." He says he tried to explain to Greg that it was just a parody.
He says Greg got mad that he used his "creative commons" music, even Greg provided it on his website for people to use. After he explained that to Greg, Zack says Greg said it was not comedy and that Zack was just being an asshole by stepping all over his beliefs publicly.
Zack says he tried to explain that it was just a parody and that Greg even parodied him before and other people.
Zack says in Greg's video, Greg said that he tried to tell Zack how he felt and Zack "didn't give a fuck". Zack confirms that he did tell Greg that he "didn't give a fuck", but that was in response to Greg saying, "I hate you. I don't want anything to do with you. I don't want to talk to you."
Zack says he responded, "I don't give a fuck. I don't hate you." He said what Greg said was the truth, but he did not tell the whole truth.
Zack says he just wanted to get to know Greg and he was trying to be polite and Greg wouldn't give him the chance. He said Greg refused to try to understand where Zack was coming from and called him a "scum bag" and an "asshole" and that he hated him. Zack tells his followers not to hate on Onision. He said he if wanted them to attack him, he would have uploaded it to his main channel. He says he just wants to vent and to get his story out there. (Source)
In 2013, Greg and Zack were both chosen to join Prank Tour, which was meant to be a group of YouTube "pranksters" traveling around the country pranking people for YouTube videos. Greg and Zack attempted to use their past friction to prank viewers and fellow members of Prank Tour.
Greg posted a long blog entry depicting a fake altercation he got into with Zack the night before. (Source) Which lead to the video "ZACK ASSAULTED ME", where cameras follow Greg and Zack fighting until it was reveled to all be a prank. ($ource)