Why is the poster of The Theory of Everything rotated 90 degrees clockwise?

I haven't found any solid reasons from the designers or people who worked on the film, but here are a couple theories that fit my thoughts as well:

I love that the movie poster is turned sideways, just like Stephen Hawking has done to various theories of physics and science.

Not only did he turn scientific theories on their sides, he also turned all expectations for his life and life expectancy on their sides. Remember when the doctor said "I'm afraid average life expectancy is two years."? He's still here 50 years later.

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To add to the slow-paced marketing for this title, which is the right way to do it, we’ve got a new poster that flips proceedings on their side and reflects the changes in circumstances to come.

As a true cross-continental hero, the Hawking biopic is sure to have fans on both sides of the pond.

As a result, you have to remember that the standard movie poster setup in Europe is landscape (though they certainly have plenty of portrait posters as well):

Whereas the standard movie poster setup in the US is portrait:

This poster can be used simultaneously in both locale's with a near-identical effect; that the film is about a man who turned the world on its side with his brilliant mind and his ability to overcome a debilitating disease.

So, I would say that it was a wonderful artistic view that also solves a production/distribution challenge.

Jane is helping Stephen to stand up

Please note that Stephen Hawking is (on this perspective) down: while he is in fact standing on the ground, he looks like he is lying on his back. What's more, he looks quite helpless: please take note at his arms- they are close to his body as if he had no strength to lift them up. On the other side Jane is above, her arms outstretched, reaching out to her husband as if trying to help him to get up.

This represents the role that Hawking's wife was playing in the movie: helping him to - mentally and physically- to "stand up" and face the life in face of the crippling sickness.