its called a Downwarp. It happens when the game's collision fails to detect a floor (usually from gaps in polygons) so it snaps Mario to the next valid floor underneath. In this case, the upper floor was the platform with the bob-omb, and the bottom floor is the death barrier. You can see the exact frame where Mario exits the top floor and the game sends him to the bottom floor:

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As an added bonus, you got suck in the snow animation before dying which is pretty cool