This is somewhat similar to the recent bug with fire elementals in Skald that came through with the last patch. Basically, the nodes that spawn the collapsing voidwalkers in Hellfire Penninsula have a really buggy respawn rate right now. Talking about the spawn points between roughly 75 65 down to 79 78
We're still trying to work out the kinks but it seems that the faster they are killed and the more people are on the different nodes, the faster they spawn at each node. At our peak before we breaked to repair we were having 5 spawn roughly every 10-15 seconds. Also, the drop rate for motes of shadow are either bugged or extremely high these mobs, as we (2 healers, 5 dps seperated over 3 spawn points) farmed 84 primal shadows, ~15 greens, and 2 boe blues in roughly 45 minutes
The only downside is that while these mobs have ~4k health, when they die they explode doing ~1500 shadow dmg to everyone so a healer is imperative (at lvl 70 you usually resist at least half of them) but it can build up fast.
edit: seems that the nodes are tied to each other, you need to kill mobs at one node to spawn them at the other node, vice versa to get them to respawn back... more people you have killing them faster, the faster the grind goes so the loot is sorta exponential