Since ETFs are derivatively priced, it's irrational on the face of it. If I sell you a bundle of 100 $1 bills for $50, it's irrational. ETFs are wrappers around stacks of stocks.
not unless the ETF leads the basket of stocks down. For example, futures often lead cash, why can't ETFs lead their constituents ? if stocks are about to go down 50%, there is nothing automatically irrational about ETFs on those stocks going down 50% first. But, as I said elsewhere, if you think it is irrational, then buy the ETF