The Mystery of the 5th Century Sarnath Buddha

When it comes to Buddhist art, one of the first thing that comes to mind is the Gandharan form which developed when Classical Greece met Buddhism in the Af-Pak region; it was a Big Bang moment in Buddhist art. Less mentioned is a major breakthrough which happened in 5th century Sarnath — the place where Buddha gave his first sermon — when a new style of representing Buddha was created. The origins of this style still remain a mystery.
Compared to other representations of Buddha, the Sarnath Buddha (see pic) is quite different. He is seen wearing a see through dress which covers his torso and has no folds; most other styles show dress with folds. The second point is not quite clear in the photo, but the left knee is a bit bent. Third, his genitals are hidden. Also, the eyes look down and he looks feminine. This unique style spread to rest of the Buddhist world — to China, to Vietnam, to Cambodia.
To put this in perspective, look at Bala Buddha (125 CE), one of the important anthropomorphic representations of Buddha, found in nearby Mathura. The statue is 9ft tall and he is staring right at you. Also his genitals are not hidden; the pose is quite strong and powerful. He wears a underskirt and exposes his torso. This is not surprising since Ananda Coomaraswamy found that the inspiration for the Bala Buddha came from the Hindu iconography for the Yaksha. You can see similar pose for a 5th century Vishnu as well. Now if you go back to the Sarnath Buddha (see pic) you can see that all the manliness has been drained out.
What exactly happened to trigger such a change? Was there a political situation which caused Buddhists to change their representation or was it in response to an ascendant Hinduism? (Note that while this change was happening, the Gupta empire was in political turmoil). Is this a feminine representation to come up with something like the ardhanari concept? Or is this a boyish look to appeal to women or queens who were Buddhists ?
Or is there any other theory?
Notes:

1. Recently I attended a lecture by Prof. Robert L. Brown of UCLA on this topic. This post comes from the lecture notes.