The stopped video shows that they were connected- I can only think of a few ways to topple something that weighs 4 tons and none of them is a bunch of people. Also, a row of large stone is between the stacked containers and the river, and the ground hasn't been disturbed- IMO, best bet is that the backhoes were left in the area and were used to topple the containers.
The operation to use shipping containers to close a 1,000-foot gap in the U.S.-Mexico border wall along the Yuma sector has suffered a brief setback - as two stacked containers somehow toppled over.
kyma.com