While riding and turning on either side, left or right, bike (handlebar) seems to be pulled to that side unexpectedly causing a oversteering.
Sandstorm tires are not compatible with the bike weight. Tires max pressure stated on the tires and is 1.4 bar (20psi).
Putting more air in, makes the problem go away. But you will have to go over the max tires limit (1.4 bar / 20psi). I pumped them to 1.8 bar and the oversteering is now gone.
But pumping the tires above the upper limit is not a safe thing to do, and it definitely makes them more likely to puncture.
So better option would be to upgrade to a better set of tires.