The high volume is mainly fund flow balancing, recent new 'inflows' by the issuing bank that sponsors the NUGT (what all these funds do on a daily basis with inflows-outflows)... But certainly is an indicator of recent of huge selling of gold miners (GDX) & a sign of upcoming selling exhaustion in short term.
Investors simply like shorting NUGT as opposed to buying DUST. Yes, the dollar is up, and gold slightly, but remember minors lose money. Mining gold is a failed business, your almost guaranteed to lose money, even if gold rises substantially. THE GDX simply reflects that.