Imagine a metallic spheroid that is riddled with Swiss cheese like holes and chambers. Like a sponge that has more open air than material. Then fill in all the open areas with ice and frozen gases. As that object passes through space, it is heated occasionally, and irregularly, by light sources coming at different angles.

The ice melts irregularly.

The steam doesn't uniformly emanate from all sides. It comes off one direction only. And, because the ice ball has a core of holey metal acting like multiple rocket nozzles, it jets out steam which changes the velocity and trajectory of the comet.

That's all 3AI/Atlas is. It's a comet whose composition causes irregular melting patterns that cause it to travel erratically.