The negative price can either be from subsidized renewables that are passing some of that subsidy onto people who use the power (a good thing!) or remaining coal plants that can't flex fast enough and would rather pay than incur extra costs from flexing (which is good in that they are being effectively fined for lack of flexibility but bad in that they are inflexible and still generating dirty power while more flexible renewables curtail).

If clean energy is being curtailed increasing usage is a better response which negative prices also incentivize but in many jurisdictions they refuse to pass those price signals onto users because they'd rather waste clean energy so they can sell them dirty energy later.