There is many factors playing into this and it's to early to say for sure.
Just remember that according to the rules of the road the fishingvessel has the right of way when fishing. To complicate things further, many small fishingvessels actually don't man the wheelhouse all the time 'cause they have to help out working on deck preparing the catch on their way in.
And in the end you always have the fact that in many of these cases you have a small fishing boat of wood or fiberglass in relatively heavy seas. This makes it a har "target" to sea for radars (especially newer ones with a lot of fancy clutter functions and videoprsessing) and the small fishingboats don't have AIS.
A further cause for this might be a watchstander problem on the tanker with problems ranging from not vigilant enough (or present) lookouts, fatigue, training, work ethics, work load on the mates or a combination of all of the above.
This is all to early to say anyhting about it, except for that it is a tragic accident for all involved. :-(
