I experience recurrent episodes of derealization/depersonalization, especially when I'm overwhelmed like being in a large crowd, but it also happens when I am alone watching a movie for example, where I detach from reality and everything becomes foggy and blurry and noisy and ... unreal. I read a little about it, and it's believed that it is caused by emotional abuse in childhood, which makes sense to me, because as a child I wasn't able to defend myself and say anything when I was abused, and I had to suppress all these feelings of anger and sadness. Now, they are manifesting in different forms: depression, anxiety, derealization, ... etc.