[sithlord]

This is usually related to drivers who do not use the cover of their truck they are legally supposed to. So rocks fly out the top.

[londons_explore]

And usually because the truck is over full too. For almost any load, if you fill the truck to the brim you have overloaded it. (Unless you're moving styrofoam)

[Tempest1981]

Or dump trucks, which leak out the seams as they go over bumps

[whaleofatw2022]

Or smaller contractor type trucks with tools in them.

Ever have to dodge an axe at 35MPH? Not fun.

[arcanemachiner]

Also mud flaps