The quick and decided decline suggests institutional selling. Maybe their valuation models reflected a 0.50% cut already priced in, so at 0.25% stocks were overvalued? After the institutional selloff, herd behaviour and panic took over reason?
There's no panic's anymore because the fed has told everyone that it is here to gurantee your prosperity and to underwrite all credit risk. They should have a hot line where we send in all of our bad debt and it is banished forever!