@Galuade@snouts.online Basically on levels where the exit is a linedef (like a box you step into instead of a switch you interact with), a dead player can slide into the exit and start the next level with a new game inventory. This is called a "death slide" or a "suicide exit".
Depending on the source port you're playing you might have also enabled a pistol start preference, or hit a function key bound to that action.