What do you think happened between Johnny Depp and Amber Heard to cause their relationship to become so toxic and end up in disaster?

The specifics don't actually matter. people are abusive because they have experienced trauma, and then perpetuate their trauma on others. We don't need the gory details. Amber Heard probably comes from abuse in some way herself, and that inflicted itself upon her relationship with Depp. Depp probably has some complex trauma as well pre-dating that relationship. People with trauma histories have a natural knack for finding each other and re-creating familiar trauma patterns. That's just the nature of trauma.