Mounting a 125mm on a 30 tonne tank isn't practical. Also, the ZTQ is rumoured to have a bustle loader which is only possible to fit on such a small tank if it was chambered for 105mm. Take a look at the Leclerc and Type 10 for an indication of how big bustle loaders for 120mm are. Tanks can also store more rounds of 105mm than 125mm so in a remote and isolated region like Tibet where reliable resupply during wartime is difficult, it's prudent to have more rounds organic to the vehicle.
Anyhow, on the Tibetan plateau, the ZTQ won't be facing any threats that its 105mm can't handle. India is unable to transport MBTs up there so if they want to invade China, they'll be constrained to vehicles such as the CV9035 which the ZTQ can handle with ease.
As to why is China concerned with an Indian invasion,; she's not. It simple pays to be prepared for even the most unlikely of contingencies.