One possible explanation would be that it detected the unicyclists as pedestrians, assumed that it would be able to overtake them, and was unable to do but kept

One possible explanation would be that it detected the unicyclists as pedestrians, assumed that it would be able to overtake them, and was unable to do but kept trying to overtake them. Seems like a failure of their path planning in this case, the same way if someone were to try to overtake on a two lane road, one would have to abort the overtake if the vehicle one is trying to pass keeps on accelerating such that overtaking isn't safe anymore.