Im not sure, could be one or more of the following:
1. Good ol' player reports, yeah some players are notorious /who'ers
2. Gold earned per day, dungeons done per day or mobs killed per day.
3. Deposit 5k per day to a guild bank, like which legit player would ever deposit that much gold to a gbank and not just have it on his character? Gold in gbanks is only used by bots to make the gold more "secure". Granted my guilds were lvl 25, but I dont think blizzard scripts cares about that.
Regarding 1: I really doubt reports would lead to synced ban on 10+ accounts.
I am 99% sure, the server auto-flags&bans the accounts, so have to be account behavior issue, which triggered the flag(s).
2. Gold per day should be not an issue, I still run some PP rogues with effective up-time 6-10/24, netting 5-8k/day, and they are still up after 2+months.
Dungeons per day could be some flag for real, especially repeatable ones, but I guess you were running random 5man HC, or predefined ones?
Mobs/day could be or not an issue - try running LFR 10hours daily for a week - Im pretty sure, you wont get a ban. Maybe they have different pools for mob kills, who knows
3. Guild withdraws/deposits are generally related to manual bans, people doing too much GBank legit transactions for scripts to filter them out.
I must add two points here:
4. If I am not wrong, the bots were aimed at single task only: farming dungeons - from the blizzard 's point of view - they are doing a single behavior - run 5man dungeons, no questing, no dailies, no raids, no scenarios, no pvp etc.
I see this pattern responsible for rush banning the majority of the dungeon/gather bots recently (Since 2014)
Any bot, aimed at 2 or more off the above tasks have much higher longevity!
5. Multibotting from single machine - the pattern is obvious:
Any common dungeon/gather/grind bot is lasting much longer if it do NOT share the same behavior (aka herbi/ore/dungeon farm) on the same machine (No matter physical or virtual) in the same time - the IP address doesnt matter.
And the ban chance is jumping to the heavens, if you bot with several characters in the same time, aka 5man dungeons or 6-7 gather bots, etc.
Have to add here your ban history too: If you have received auto-bans recently, the next ban comes much faster, with the very same botting behavior as before.
The above can be explained by the Blizzard's own wow installations/Windows registry, used for identification on every wow client, ran on the machine, and link them by certain criteria s. I am pretty sure, this is already scripted and heavily used by Blizzard to reveal and target multi-botters (Not the famous iP/MAC address, hwid or anything out of Blizzards authority)
P.S. I guess, you are running every 5man party on a dedicated machine, so they fell in the point 5?