I believe what happened is this:
About 24 hours ago, Indians crossed the LAC at night and attacked small Chinese unit.
PLA reinforcements came and attacked the Indians, inflicting heavy casualties including killing the colonel.
The entire Indian company was wiped out. The Indians had no idea what happened until the morning (around 16 hours ago). When they got a phone call from the Chinese saying "come collect your bodies" they escalated.
There was an artillery exchange about 8 hours ago. This was when the news about the dead colonel was released.
More and more Indians will be reported dead within the next few hours or days. The artillery exchanges will continue.