Only so much storage capacity for crude oil (think huge tank farms) and we have already been over stocked for some time now. Because you can’t just shut off a producing oil well (costly and risky), you could conceivably get to a scenario where demand has fallen so sharply that we fill all available storage as the existing wells keep pumping oil.

At that point, the oil has to go somewhere, so you are paying someone to take it for you at less than the cost of putting it in temporary storage or disposing it, therefore becoming a lesser of evils/costs you are forced to deal with. Hence, “negative” price. This is pretty extreme but is conceptually possible.