My bunny does that when she's scared or when she is mad at me for some reason, I found this website very useful to help me understand bunnies' body language: http://language.rabbitspeak.com/
Hope this helps.
You have to sort of take all of the rabbit's body language as a whole, in context, to figure out what any particular thing means.
Having the ears back like this means, "I'm really relaxed, so much so that I don't really need to listen for anything bad."
This is especially true if combined with the "feet stretched back" pose and eyes half closed:
The "I'm scared" signal would be accompanied by an extremely alert body posture, ready to spring into action, with the eyes wide open and round, and the ears would more likely be up and alert than lying down:
Ears flat could also be aggressive, if accompanied by growling or boxing, I'm told - but I don't have any pictures of that, since I've never seen it.
It would really depend as mentioned before how their whole body looks not just the ears.When mine get annoyed they put one ear back slightly and I know to leave her alone or I get nipped at or thumped at.
My female netherland dwarf does that a lot and like others have said it depends on her body language. I can also tell if she wants to be petted just by her ears she will relax them when she wants to be petted if she doesn't want to be petted she holds them really tight to her head.
She loves me very much too and she'll even start licking my face at the same time when her ears go down when I'm holding her. Always when I go to pet her, her ears go down. There's never a moment when she lifts her ears when I'm petting her really.
My bun will come up to me with her ears up and I know she wants a pat, but when I go to pick her up she tenses her body and starts to growl and kick until I hold her close to me and then she completely relaxes again. I think it just comes down to knowing your bunny