After observing the standoff situation and the responds from both sides, I think it is very likely that this a plot by the USA and India to disrupt One Belt One road.

Reasons:

USA:

This evil empire has always trying to prevent China from expanding its influrence such as the South China Sea & East China Sea. USA wants to contain China but wish not to do it directly and openly. So USA encourages and supports India, Japan, Vietnam and Australia to counter China. USA welcome more countries against China.

 India:

India is very unhappy China and Pakistan are constructing the Pakistan corridon which pass through Kashmir and Aksai Chin. Also its passage to the North East Frontier is under direct threat from China if a road is build near the area. India is more than happy to have USA at her back to stand up with China. That's why India dared to send army into China territory.