Can someone remind me how MC betrayed ML in a previous life?

the ML is a reincarnation too but I think he had his own agenda in his second life and that's when the mc "betrayed him" so his third life he's trying to change the outcome. Ultimately I think it's a misunderstanding on both parts that they betrayed one another but then again who knows it wasn't really said how he was betrayed (if I remember) but just that he was betrayed