1. Anxiety 2. Sexual 3. Dominance 4. Play 5. Self-soothing
I suspect that this is sexual for Sophie since she consistently humps the one "toy".
I wouldn't be concerned about it myself, but if it bothers you, then preventing access to her Boyfriend should solve the problem.
I had a neutered, male cat some years ago who did the same thing but only to one particular toy. It didn't matter if that toy had been out of his sight for months, as soon as we took it down from the shelf where we kept it, he would mount it....and I have to admit that it was pretty amusing:)
I hope this helps.
I guess I should have told you that she has been spayed. Does this make a difference as to whether her motivation is sexual?
Pam: Sorry but I meant to include this in my answer...... It doesn't matter whether she has been spayed or not just like it doesn't matter with males whether they've been altered or not. Dogs can still masturbate even though they've been spayed/neutered and they frequently do! So if this is sexual for her, then this is probably what she's doing (I hope you don't mind me saying so offensive) Deb