Leo complained that the group assignment was nothing but busy work and also that he was the only one doing any work. Based on this information, why is social loafing happening

If Leo is the only one doing any work whatsoever, the other members of his group are social loafing. Leo is also apart of the problem if he only complaining and not convincing his members to work all together. As a result, less work will be done.

Explanation:

Social Loafing: In social psychology, social loafing is the phenomenon of a person exerting less effort to achieve a goal when they work in a group than when working alone and is seen as one of the main reasons groups are sometimes less productive than the combined performance of their members working as individuals. (Source: Wikipedia)