Negative voltage to the drain of NMOS
For the 4-terminal NMOS that you used in your simulation, there is a PN junction between the transistor body (P type) and both the source and drain (each is N type).
So, when you bring the voltage on the drain below ground, with the body still connected at ground, you will forward bias the PN junction from the body to the drain. Like any other forward-biased PN junction, large currents will flow.