RayTech mentioned cardboard and he is spot on about it being a major player, but there are other significant ingredients too.
It is a mix of many things blended together dynamically by the machinations of the equipment.
Cardboard boxes and separator sheets , wooden or composite pallets....all shed tiny bits of material when brushed against surfaces such as a concrete floor, or when rubbed against other units of the same material. The rubbing or brushing action often imparts a static charge into these tiny filaments or dust motes which can make them attractive to each other, and to other airborne dust and particles.
The more a pallet or box gets scrubbed across the floor, or dragged against a neighboring box or pallet, the greater the amount of material will be removed from it in the form of these microscopic "shavings".
Then you have the dynamic of the forklift tires themselves doing their part to create the ubiquitous black dust so prevalent in warehouse and freight dock venues.
The tires shed material constantly even when under the control of a careful and conscientious operator (quite a rare bird if I might add) simply by erasing itself when the tire contact patch flexes against the floor as the tire rolls. Jack rabbit starts and nose dive braking increase the loss of tire rubber.
Having to do a lot of steering to navigate around obstacles increases the loss of rubber on steer axle tires.

Toss in a few small chips of wood from pallets or shreds of cardboard/paper/other at random spots on the floor in the traffic lanes for the forklifts to pulverize and you have quite a lot of fine particulate matter work with.

Now, drive the forklifts back and forth across all this stuff so that the pressure of the tires further grinds it to a powder-like consistency and imparts a carbon black color to it (from the tire material).
Keep the stuff stirred up and airborne as much as possible with the passage of forklifts and it will settle on objects everywhere in the area/building.
A small operation with a limited number of electric trucks may not generate a lot of this "product", but a large, 24/7 operation with a lot of ICE powered trucks will actually generate a great volume of it in a day's time.

We run 90+ ICE trucks 24/7 and I estimate (loose calculation based on what I see collected by our sweeper) we generate around 20 to 30 gallons volume of this dry material DAILY. It may be greater than that in reality but I can often clean nearly a quart volume of dust out of the dust filter on our Tennant 6600 sweeper in just a single "shaking" of the filter.
We run the sweeper around 20 hours a day on average, and the operators are supposed to shake the filter at about 6 to 10 minute intervals.