What could explain 1998 KY26’s unusual acceleration?

An apparent acceleration does not automatically mean the object is being pushed by something exotic. It can also come from limits in the observations or in the model used to predict its orbit. For a very small object, tiny forces can matter, so researchers would want to check the measurements and account for effects such as sunlight heating and radiation pressure before settling on a cause.

Outgassing is a reasonable idea to test, since material escaping from a surface can act like a weak thruster. But if the new analysis finds that explanation does not match the measured motion, that narrows the options rather than solving the mystery by itself. The useful next step is more precise tracking over time, ideally alongside observations that could reveal activity at the surface.

I’d be cautious about the word “unexplained” here. It may just mean the available observations do not yet distinguish between several small effects. At roughly this size, the object’s physical properties and rotation could matter a lot, but the short description alone isn’t enough to say which explanation is favored.