Negative central bank rates are an attempt to spur consumption via all of the stimulus created since the financial crisis. Banks have been loath to pass on these negative rates to depositors because no one wants to be the first bank to do so, as depositors would flee that bank for another bank that was still eating the negative interest rate. However, the banks legally cannot coordinate actions as that would be evidence of cartel behavior. So they have just eaten the losses thus far.

This appears to be a first step to pass negative rates onto consumers, in this case onto borrowers where the first mover has an advantage.