Multiboxing is botting in real time, basically.
There's never just one thing affecting markets. It's a living system. I think all of the reasons mentioned here have merit. A lot of people came back for the XP buff, but are fatigued again, and are probably dumping inventory before unsubbing just in case it becomes worthless in Shadowlands. Which IMO it won't, because previous tiers of materials still hold value. In many cases, especially ore, more than they did towards the end of their expansions.
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Right now the volume is too great on a lot of things. People are dumping stuff. On my server, there are several hundred thousand Zin'anthid at 6 gold.
Either it's people dumping before unsubbing to return in Shadowlands, or it's the result of boxers and botters.