Because she is being really naughty tonight! DS has had his imaginary friend for two years, lately she has been the one who gets blamed for naughtiness, and tonight she is 'teasing' him and tickling him.... I am tempted to go and 'tell' her she's not allowed to play here any more, but would that traumatize DS?
(Please note, I am fully aware that the imaginary friend is just that so any woo-bollocks suggestions will be greeted with a swift kick in the fanjo)