De Niro and his former wife, Grace Hightower, entered a 'divorce' prenup agreement in 2004, seven years into their marriage. They already had a 'marriage' prenup agreement in place, which the new 'divorce' prenup amended. First, there are lots of things covered under that agreement that will never see the light of day in the press. Secondly, in any marriage-divorce situation, there is an emotional attachment to almost everything, which is the source of irrational behavior among the parties involved.
There is a mechanism in place which allows De Niro to reduce payments in times of financial shortfall, which De Niro is now acting upon. For now, the judge is allowing a 50% reduction in monthly payments, but also ordered that De Niro put up money for a summer residence for Hightower and kids since their regular summer residence is being denied.
From a regular person's standpoint, it is hard to generate any sympathy for people with that much money. But in relative terms, if someone with a budget of $100,000/month suddenly finds themselves only receiving $50,000/month, that leaves a $50,000 shortfall in their monthly expenses. This would be very difficult for any person to handle.
In 2004, Hightower gave De Niro an out, and De Niro is now taking it. That's on her. She agreed to it. She should have tied in net worth, but she didn't. And as long as De Niro can keep his annual income under $15 million, her payments get reduced.
De Niro may be a POS, but this is about a legally binding contract both parties signed off on.