Actually, I can tell you exactly how that happened. The truck died on the road and wouldn't start so they were going to pull it out of the way. When they hooked the tow strap to it and also an airline to air up the truck and trailer so they could release the brakes, they didn't release them, or didn't give time for them to release before the first truck started going.
So what they had was an 80 000lb frac pumper of dead weight sitting on 5 axles of fully applied brakes + 550HP pulling truck which also weighed 80 000 lbs (also a pumper of the same dimensions) with a slack tow strap = yoink!! And an OH SHIT exclamation from the supervisor!!
Gotta love the oilfield!!