The percentage of contracts that result in delivery is only a marginal factor in this case. The fact that physical delivery exists makes it so that positions must be closed out; even if the May contract is down 44% on the day and you thought you would be clever buying the dip on Friday when oil couldn't possibly go lower.

That’s true but not really relevant here. if traders are holding on to any open contracts for May past tomorrow, then I believe they will have to take physical delivery.

Thus, traders who still have contracts open but cannot take physical delivery will have to close their positions out today or tomorrow, thus causing the dip we are seeing.