Your brain uses a combination of outputs to determine your orientation and keep your balance, this includes mostly the inner ear, vision and the sense of where your limbs are (proprioception).

When those inputs differ, your brain tries to compensate. And often it just can't. Also, one of the things that could make these inputs differ is through some forms of poisoning (I don't really remember how exactly). This is why we developed or survived better with the mechanism of puking when this happens.