This is most likely orange or "unclaimed" mobs. Any healer that experienced natalan wolf camp is quite familiar with this. All the tank needs to do is pop any skill, doesn't even need to be a enmity skill to claim the links and the healer should be safe. But if any mobs are orange and not red, the are coming for you, healers.
When mobs link it happens after the initial agroing action so effectively it's as if nobody has done any action and therefore they remain unclaimed. The first person to use a skill at this point will get claim and thus also get the agro. This is a simple base fact, on top of that, enmity generation then comes into play. If a BRD pops raging strikes he will get claim and agro, but it does not generate as much hate as a cure, so the healer will quickly strip them off.
A lot of the time when tanks complain that people are attacking before they have hate on all the mobs it's simply a buff action the DD have used that takes the initial claim which then causes their flash to miss and all hell to break lose.
Back when we were doing 12 mob pulls in 1.0 at Natalan my tank's rotation was to run around, gather all the links then use something stupid like Featherfoot, Sentinel or any useless skill to turn them all red on him. Then I could regen and by the time he needed a cure he had more than enough hate. Which is another thing that should be mentioned, a healer cannot regen you until you have hate either. Tanks should keep an eye out for a lingering regen before pulling a large pack.
