Apparently mainly air pressure:

> The pressure wave created by an arc flash explosion has the force of thousands of pounds per square inch. This is powerful enough to knock down or throw nearby workers and at the same time cause damage to the eardrums, lungs, brain and other vital organs.

https://www.electricityforum.com/iep/arc-flash/electrical-ar...

(I didn't know that, and just found that explanation while looking for an answer to your question.)

A different risk is that electric current passing through the body could cause muscle convulsions, which could also cause someone to move or fall suddenly or erratically.

The heat of the arc rapidly expands air (like a lighting bolt), the heat of the arc also vaporizes the metal conductor, and the vaporized metal expands rapidly, and enormously.

It's basically a small explosion.

The hot metal then ignites in air adding even more heat to the explosion.

There is an explosive heating of air that’s like a bomb going off.