Originally Posted by
Sephnoir
Imo, probably has something to do with being flagged for the raid mob in question.
A loose example would be being flagged for pvp after you heal/rez a flagged ally, or damage an enemy.
So I'm guessing you are flagged for lockout if you attack it, or assist an ally who is flagged by the encounter. Though, since you are still in a raid group in the same instance, you can roll on loot that drops, despite not being flagged for the boss.
A good question though would be; that since now most player buffs act as auras in cast while in party(a lot of them disappear if the caster of said buff leaves the party/raid) could your afk friend get locked out if he/she buffed you inside the instance.