Ok, i got my temp ban too.
I can't reproduce my ticket here because it's in french but i contested my ban, they reply that the suspension was not going to be removed.
I send an other ticket, asking for more information and the gm tell me that multiboxing is ok but it seems that one of my character was not participing activly in some battlegrounds.
The character who was ban was on a tank half-pve spec (i was thinking that keep one char to tank the boss was a nice move) so it's probable that he get a lot less HK than the others.
So maybe the auto-ban is a system using not only the afk report but the performance of the character.

Maybe that a character always on follow, with lot of report and bad hk ratio trigger a detection routine.