SheCycles.com:

So how does yoga fit into cycling?

Well it works on a good many levels. First take the body, cycling is great for keeping fit and keeping the heart healthy and working and toning certain muscles. As with all sports, certain muscles work hard, and others don’t. This invariably means that we suffer from muscular imbalances which then have an impact on our skeleton and other muscles. So for example, tight hamstrings caused by cycling can cause lower back pain. Cycling keeps our hips flexible in one direction, but cyclists who try and sit with their feet together and bring the knees to the floor soon notice how tight the hip flexors are in the other direction, which may lead to all sorts of hip problems later in life. Being hunched forward over the bars causes the chest to close and tension in the shoulders, leading to poor posture which also has a knock on effect on our breathing.

via Making It Easier