Why has this happened...... Don't look if you don't like blood

As every night I feed all my geckos and handle all 8 but last night when I picked up Speedy she wasn't happy about something as she jump of my hand and as she hit the floor her tail came off it wasn't that high I was sat on the floor. Don't understand what she felt threatened by so badly this happened she had a good size tail too.

I've cleaned it and put some animal disinfectant on it. She will regrow it no problem it will just effect her on balance for a while