One possible reason could be to show Cato's physical strength. In many fanfics, characters' physical abilities are exaggerated or highlighted in certain situations. Cato might be trying to intimidate Thresh, or it could be part of a fight scene where he gains the upper hand by lifting Thresh off the ground.
In some fanfics, such an action might be a plot device to create tension or drama. It could lead to a turning point in the relationship between Cato and Thresh. For instance, it might make Thresh more determined to fight back, or it could be the start of an unexpected alliance if they both realize they have a common enemy.