It deviated from a Keplerian orbit. It increased speed and changed direction. The deviation was with super-high statistical significance (30 standard deviations). Harvard researchers calculate that one explanation is that the asteroid was an alien solar sail.
Harvard researchers Bialy and Loeb offer a counter-explanation to the comet theory. If 'Oumuamua were in fact a comet, why then did it not experience outgassing when it was closest to our Sun? In addition, they cite other research that showed that if outgassing were responsible for the acceleration, it would have also caused a rapid evolution in 'Oumuamua's spin (which was not observed).
They calculate that the movement is explained if was a light sail with sub-millimeter thickness.