Remember Klein? Good old butler of the Schnees? Last saw him helping Weiss escape.

I have something of a theory.

I don’t think Jacques is dense; he’s probably well aware of Weiss’s familial relationship with Klein. I don’t think he’d have to do too much mental gymnastics to figure out Klein would help her escape, and he wouldn’t be happy about his orders being disobeyed.

So, what to do with someone you brought into your home and paid a decent wage, only to ‘betray your trust’ and make matters worse for you? Well, if you’re Jacques, you punish them. You send guy who alludes to the Seven Dwarves down the mines so the backstabber can serve you in a less cushy job. After all, he didn’t seem to respect or appreciate the job he had before.

TL:DR - Jacques figured out Klein helped Weiss, got mad, and banished him to the dust mines as punishment, fulfilling the miner imagery of the Seven Dwarves.