Why did Rosie Huntington Whiteley pose with her hands behind her back so much during her time in Transformers?

Having just looked up her “images” on Google, I think you are mistaken. Plenty of pictures of her with her hands in full view, often as a key element in the picture.

There are lots of images with her hands behind her back during this time.

Why would any of us know the reason?

She was hiding a toaster.

That sort of pose, hands behind the back, can emphasize one’s figure.

Wow, she wears god-awful amounts of lipstick, bleh!

I think she’s beautiful