>Assuming the competitors are currently or have incurred expense in dealing with it, the only reason left (as I see it) would be altruism on their part in banding together with a competitor.

Its not just altruism. If you expend resources on something, you want to get the most value out of it as possible. It would be extending the value of their investment into lawyers/settlement/etc. to band together and get a possible reversal of judgement/recompense. There is definitely something to gain from taking down the troll, including recouping any initial settlement.