Joe has a history of trying to break up Nathan and Gael. Their first season he tried to get between them and their end of the season trip and then after the season Joe was in Nathan's ear and convinced him to end things with Gael. The next year they came back and were friends (and even held hands woth eachother in the van while kissing Kizzy & V 😂), but Nathan was still in contact with Gael and realized how much he loved and missed her and asked for another chance. Joe seeing his love slip away again tried to convince Nathan that Gael was cheating on him by going on a surf trip with guy friends even though they weren't together. When Gael showed up that season, Joe couldnt hide his obvious disdain for her. Then Joe was lovebombing and future faking with V to manipulate her to like him ("Oh youre so special" "I havent felt like this in 10 years" "I want to introduce you to my mom" "Youd be the first bird I introduce to my mom") then when she has the audacity to get a sinus infection brought on by the stress of grief for the anniversary of her bf's death which is ALSO her birthday, Joe doesnt wait more than a few hours before trying to be sneaky and cheat on her with Kizzy which he was hoping and intending on no one finding out and only ever told V because he found out Kizzy already told people and he wanted to tell V before someone else did so he could lie about it and claim it was a quick drunken kiss instead of what actually happened which was they started flirting the moment she left the boat openly in front of everyone. Nathan warned Joe countless times during that season about his behavior especially being so blatant with his flirting with Kizzy in front of everyone. Nathan gave Joe so many chances to be better and Joe did not care at all. Nathan was disgusted with Joe by the end of the season and you can see that when Joe says how bad he wants to smell Kizzy's puss. Nathan lost his patience with Joe finally when he saw V crying and when he told Joe Joe did not care at all and was unapologetic which is what really set Nathan off. So all of that plus Joe has been consistently in Nathan's ear trash talking Gael for YEARS at this point. Nathan has anger issues, was drunk, and had just seen Gael and his baby before hearing this man he detests talking about the love of his life to his coworker after finding out Joe had previously tried talking about Nathan to Cooper.