Long covering, actually. (Here is the long who is forced to sell at a negative price to get rid of their long position.)
No demand, surplus inventory already out there, so taking possession in May requires storing them indefinitely.
Investors decided to factor that cost in when buying these.
You can close your position by buying the month after futures. This will require you to store crude oil for just one month. I think the issue is that storage facilities are almost booked out and the remaining capacity is just so expensive.
“oil firms have resorted to renting tankers to store the surplus supply and that has forced the price of US oil into negative territory”
It's entirely possible that no barrels will be effectively sold at a negative number. Because even after expiration you can negotiate OTC, it depends. At the very least, the final print likely will be renegotiated.