Banks are sitting on reserves because they know how many dogshit mortgages they sold and they are aware they will not get bailed out a second time if they don’t have the reserves to cover their loan losses.
As noted in the comments the two metrics to watch are the default/foreclosure rate and the unemployment rate. Seems you could model by bank when they are most likely to need reserves when their variable rate loans reset, pushing borrowers into trouble.
Check that. Variable rate loans aren’t going to reset higher in the current near-zero interest rate regime. Banks are holding reserves indefinitely since they don’t know when interest rates will rise again and they don’t want to get caught flat-footed when they do rise again.