What causes them??????!!!!!

So two days ago was ten years since my dad died. I was upset about something earlier but got over it. Then out of nowhere I get a flashback about my dad being beaten. I could hear his screams in my head like I did as a child laying in bed at night. I didn't do anything to help. I was scared. I didn't know what to do and felt so helpless. Why didn't he take us and leave? I kept hearing the yells and I was ugly crying hard. Then the derealization set in and I was so disconnected for idk how long. I'm just now feeling somewhat normal. I hate this. If I feel like this on a random day how will I do with EMDR?!