I believe in order to make sure the pillar isn't cast on melee you need a preset number of ranged targets, similar to many other boss mechanics. If you don't have sufficient people at range then the targeting pool includes those in melee range. Haven't done any thorough testing on the particular encounter but I would imagine it requires 4 ranged targets on 10 man and 10 ranged targets on 25 man, give or take a few. If your strategy involves only the hunter standing at range then I would say this is your flaw. You should several people at range and they all have to dodge the pillar when it occurs however should still be stacked up to make sure the pillars occur in a predicable place.
Now the fact that you have seen it targeting the hunter could simply be luck based. Like I said, mechanics like this tend to have a set number of targets. For example, if he simply picks one of the four furthest targets, you'd still have a 25% chance of the hunter being picked if they were the only target at range.
Like I said though I haven't done thorough testing of this particular mechanic, I'm only basing this on similar boss mechanics we've seen in the past 6 years of the game. Such as Vile Gas from Festergut in Icecrown Citadel, in order to make sure it was never cast on melee you needed a certain number of ranged targets for him to choose from (3 in 10 man, 7 in 25 man in that particular case).
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.