His aunt wants to use ML to get access to the inheritance because the ML's grandfather is obsessed with a male heir and she has no children of her own. Both of them were super shitty to ML and he wants nothing to do with them. At the end of the first season auntie used Dohu to threaten ML into submission, instead ML disappeared from both of their lives, presumably to protect Dohu but to also free himself from the toxic family. It looks like he went far away, changed his appearance (died his hair black) and lived a low key life for a while. It's now been revealed that the auntie still stalked and harassed him which pushed him to becoming an actor. He plans to use his fame and public image to drag his family down once and for all - he has video footage showing his gramps now being demented and held against his will by the aunt, which is super illegal and would ruin the family name for good. We can assume he applied for the dissolution of the adoption to both provoke the aunt and to have evidence of distancing himself from the family as he will also be dragged down into the scandal. Auntie is now crashing out over her greedy plans failing and apparently with some feelings of guilt mixed in over her sister (ML's mom) dying a sad pathetic death.
I don't think they specified the exact reason. ML's mom eloped to be with ML's dad who turned out to be horribly abusive to both his wife and son. Mom eventually ran away and ML ended up living with the aunt but still missed his mom. The aunt told him she'd arrange a meeting with the mother and then took him to her grave and that's how we (and ML) found out about the mother's death. It was also a nice preview of how absolutely cruel the aunt can be. I think that's the gist of it but I'm also a bit fuzzy on the details.

Y’all I’m so lost what happened to the ml family drama I didn’t get it pls can someone explain aidbaidnandnd