Designing a horizontal autoloader for 125mm ain't especially hard. If anything, it's going to be mechanically simpler than one China already has.Surprising choice to go with 105mm. Maybe has got to do with the autoloader set up and design.
-China already has lighter MBT("medium tank") in form of type 96A. Here they clearly wanted something different.
-105 is riffled, 125mm is a smoothbore.
A riffled gun is basically an equal/superior option for anything other than shooting darts, and even then there is difference only if you're trying to punch through MBT frontal armor. ZTQ-15 isn't an MBT nor it is a specialized tank destroyer.
Fin-stabilized HE tend to be worse than rotation-stabilized ones(because, well, fins), and their accuracy rapidly deteriorates with range.
-105 is a standard Chinese gun for infantry support, thus any ammo development is going to be mutually beneficial.