I'm guessing this is strictly related to people using bootleg THC cartridges. You can buy empty carts on ebay/Alibaba and fill them with whatever you want. I'm sure there is a proper way to make THC vape juice and it doesn't involve using vitamin E oil. THC is soluble in vegetable glycerin which is the same component used in standard nicotine vape juice so I don't understand why Vitamin E is being used in the mixtures. Only thing that makes sense is that is being used as a thinning agent or some moron thought THC "oil" was made by actually using oil.
Since vaping flower is similar to smoking flower I would think that the problem lies in the unregulated juice that black markets provide. I haven’t heard of any cases of legal carts being the causes but they haven’t been all that forthcoming with the sources.