Unless the entire alert team is running around with all their powers off, traipsing among the civilians as they're drained, not firing off a single attack, and not aggroing the boss, then someone is dealing damage, necessarily to the boss, since Soul Siphon begins with the boss as the only enemy on the field. Just moving within the boss's aggro range will generate some hate and put whoever does it on the top of the boss's aggro table.
By "aggressively tanking the worms" I mean someone going out and purposefully rounding up their attention with attacks, ensuring that they shoot at the offtank and not the main. Naturally, if your people are standing around with their thumbs tucked in their anuses, or attacking the boss and dying like flies, then aggro's going to be all over the place if anyone tries to do anything (and your alert was over before it began).
When a worm spawns, it goes like this:
A). Who's the boss's target? (Captain Tankerton)
B). Is that person in range? (Yes.)
C). Is that person a valid target? (Yes.)
D). Attack. (Attack Captain Tankerton)
E). Standard aggro rules from here: they have their own hatelist, which starts as a copy of the boss's but then changes based on their own aggro generation or lack thereof.
Now, given that the mindworms are both ranged and immobile, there comes a few alternate situations that can crop up.
A). Who's the boss's target? (Captain Tankerton)
B). Is that person in range? (NO)
C). Are there any other players in range? (Yes - Healy Nicegal and Bloody Swordsman)
D). Which of those is higher on the boss's hatelist? (Healy Nicegal)
E). Attack (Healy Nicegal)
F). Standard aggro rules from here. They now have their own hatelist, which starts as a copy of the boss's but quickly changes based on their own aggro generation rules.
In this case, the healer gets shot right off the bat because the tank is out of range and the healer is next on the hatelist. Then there's this situation.
A). Who's the boss's target? (Captain Tankerton)
B). Is that person in range? (NO)
C). Are there any players in range? (NO)
D). Screw it - stand around immobile, waving around in the air. If anyone approaches, aggro based on your own hatelist.
This occurs if nobody's in range of the worms. Since they can't move to get into range, they just sit there and be the docile little annelid turrets they are. If anyone DOES wander into range, they immediately start taking potshots at the new target.
Since the spawn zone is far outside the range of any worm (they simply don't spawn out that far), then the only way to get shot by a worm when the boss isn't aggroed is to scrupulously avoid drawing aggro from the boss while you're walking around the central forum. Given that the boss wouldn't have a hatelist, the worms spawn and immediately go to their own hatelists, which basically says "shoot the nearest sorry chump in range until aggro generation says otherwise."
