This is a vanilla bug on VH/Legendary, but is more obvious when the chaos invasion script has been edited to spawn more armies. For some reason, a large number of Warriors of Chaos armies will stay at their spawn. Only when a properly functioning army has been destroyed, will another of the stationary armies then move.
To see this in action, you need only look at their spawns in Naggaroth, and The Old World. I have made a mod that jacks up the number of spawned armies, as well as adding the spawn locations that were previously used by Puppets of Chaos, and Servants to Chaos : https://steamcommunity.com/sharedfiles/filedetails/?id=2396719784
This happens in both single player, co-op campaign, and head-to-head campaign. A friend and I made it down to Lustria by the time the Chaos Invasion occurred, and were wondering why Chaos had not attacked us yet, as it had in prior campaigns before the chaos invasion script was remade by CA. We found them all clumped up by their spawn, and would not function at all. They just stood there, even if hostile armies were right next to them: https://i.imgur.com/TZheqc4.png
I did not see this issue before CA updated the chaos invasion script. Chaos would throw all their armies into the fray, as they should.
Edit: Yep, I made a workaround for the issue, which also proves this problem is with the sheer number of armies a single faction has.
By making use of the _lzd, _hef, and _def chaos factions, like we had before the chaos invasion revamp, the armies move fine. Not only that, there's no horrific freeze while they're processed, unlike when Warriors of Chaos has all the armies.