I think there is a huge difference between the truck and the taiji grand master. First the truck is a dead thing and can only sit there to be pushed. However with the taiji grandmaster, he is able to respond to the force applied on him.
Notice how grandmaster ChenXiaowang holds onto the strongman and nevr lose contact with him. In this way he can 'listen' to the strongman's force. Depending how the force is applied the taiji grand master is able to counter it by rotating and thus directing the pushing force towards the ground. The push force is redirected and converted into a spiralling force into the ground. Hence the strongman finds himself pushing against the ground.