The cause is storage capacity. Oil has to be stored, it occupies physical space, you can't just dump it in the sea. If you own futures about to expire (meaning, become deliverable) and you don't have a storage space where to take delivery, you face significant penalties. Since US infrastructure is nothing short of miserable, after having been decaying freely for thirty years, spare storage capacity in Cushing, OK is minimal, and so...
Brent for instance doesn't have the same problem. This is a well known kink of the energy market, which is precisely why they say noobs / retail shouldn't be presuming to trade in these markets : things everyone knows about happen and you're left holding a large bag of "Wtf".