What weight class are these powerlifters, approximately?

Could be body type too...like say if you are a person with relatively short legs and arms but a long torso that would cause you to have trouble at the start and typically miss off the floor. Pulling from some sort of deficit like others already mentioned would be great plus emphasize glute and lower back strength. You want to stay with an arched lower back and don't try to rely on lower back because that would be taking your weakest link and relying on it so you'd want to place a premium on strong glutes and a very stable lower back.

And you are of course right on the money in terms of explosiveness. Speed pulls will be really good if you stick on the floor. I would start with around 40 or 50% and see what that is like but most of it should end up being done 50 to 60%, imo...

BTW, it IS okay to start out a session from a deficit to "warm up" and then switch to regular deads. As long as you are proficient and sure of your positiong. It can help you as far as sort of tricking your body into pulling a bit faster from the bottom.