Companies that have stockpiles of oil are trying to get rid of them because keeping oil in long term storages are costing them storage fees and no one is buying at the moment. There is a massive amount of surplus oil in the world. US, Saudi Arabia and Russia are pumping oil like there is no tomorrow. So companies with existing supplies are willing to sell the oil at a loss or even pay to get rid of it to cut even more losses because demand is not expected to return in the next few months. Hence we got the negative price. Many oil extracting companies with high production cost will soon be filing for bankruptcy. Good time for consumers.
I had a look at this at the time.
Most of it is the "rolling over" of positions.
Eg. Selling at the negative price and then immediately buying at that same negative price.