1998 KY26 is a small (~30m) body that rotates extremely rapidly, with a period of 10.7 minutes. After the observations of Oumuamua, astronomers searched for other objects that had unexplained acceleration effects, and discovered that 1998 KY26 was within this category.
The acceleration of 1998 KY26 is, theoretically, too large to be explained by the pressure of solar radiation (which is still included as an option below). It is posited that the asteroid could be a 'dark comet'. In other words, there is material being ejected from the asteroid, but for whatever reason we haven't yet observed that (which, theoretically, we should have). This is the second option.
The final option is that there is something else going on here that isn't theroretically well-defined. This doesn't have to be aliens or new physics, it could just be an unexpected effect we haven't considered.
Hayabusa 2 will visit 1998 KY26 in 2031 but it's certainly possible that telescope observations will provide answers earlier.
1,000@JoshuaWilkes thanks for bringing this to my attention, this paper is really interesting! probably updates me slightly against 'Oumuamua having a... weird... explanation, given that there are several non-interstellar bodies with similar unexplained acceleration effects.