The smaller boat was moving under it's own power at the time, or at least moving (the wake behind it shows that), which means that it would have had at least rudimentary directional control.
The Japanese vessel would have had a much harder time trying to avoid it, than the smaller one would the Japanese ship (it takes a rather long time to turn a big, heavy boat).
It wouldn't surprise me if the Sea Shepherd crew put it on a collision course and left it idling on purpose, as IIRC they've tried ramming/intimidating the Japanese ships with their bigger boats, and they constantly try to put their smaller boats in the way of the Japanese boats to get them to change course.