This has happened occasionally in the past while on bosses, but it was particularly noticeable and annoying last night:
During last night's KF invasion, when I attacked a player and they were then attacked by 7 invasion mobs (8 attackers total), I could not turn toward my PK target and would get the message, "Your target is already fighting with 8 others". I was one of those 8! If they killed one of the 7 mobs on them, if I timed it right, I could turn to them but as soon as one of the mobs on me died, I turned away to the weakest on me again.
Annoying and frustrating. Hopefully an easy fix.