This is the 2nd time a Chinese soldier has been held by India in the past few months
Because the Indian Army is just sitting there, hunkered down. While the PLA is continually probing and expanding its operational capabilities in the area. That carries risks.
Alone by himself? Scouting/spying probably and got caught.
I doubt it, nothing is done 'solo' in the Army, not even recon. Most likely this guy just got split off from his patrol at night and wandered into enemy territory. Comms don't work well in mountainous terrain, it's easy to lose contact.
There's not much intel to be gained by India from this. They can't torture him or force him to reveal anything besides his serial number. Nor would he know much about operations, and whatever he knows (comms frequencies etc.) would have already been changed by now.
With that said, the PLA's high command is not gonna be happy about this. A lot of people are gonna get yelled at.