As we are in paranoid mode (and it's fun to be, isn't it?): not only it's unusually close, but it seems that Earth is the planet it pass closer in all the Solar System.
Maybe somebody should check if that "slightly acceleration" bring it closer or farther to Earth.
Maybe we have been photographed. I hope it's not that somebody shoot us and failed.
There is a bit of sampling bias, given that we are looking for space objects from Earth, and so it easier to detect something that comes closer to Earth. Not sure how significant the bias is for an object of this size and speed, but I'm guessing that it is probably a relevant factor.
AU stands for Astronomical unit, and it's used for measuring distances in space smaller than light-years. 1 AU is the distance between the Earth and the Sun. 0.163 AU tells you how close it came to earth.
I'd prefer to use light seconds/minutes for that. Not only it's easier to grasp (0.163 AU seems awfully close, but 81 light seconds sounds reassuringly far) but it'll probably be more helpful when we have to communicate across these distances.