It’s because paper market speculators always drive silver back to the cost of production ($26 right now) when dumps are engineered.
That is to say the banks stop selling futures at that point because they know it’s not sustainable.
They can’t get it below the cost of production for long because supply would be constrained for real in the bullion market – not just suppressed using futures related manufactured excess supply.
Easy peasy – they’ve been doing it since ’75.
Cheers