HI Virginia7,
For me it's usually when things have overwhelmed me and I lose my touch with reality - and I forget what I have been doing or where I was going or cant remember why i am wherever I am
It can also happen under other circumstances - when I was attacked there are blanks in what happened - I think sometimes your mind stops you from remembering things that are either too hard to take or are not important -
I hope this helps, - take care

MBC