When fire is given the physical properties of solid matter.
Sometimes in fiction fire is given the physical properties of solid matter. Fire is capable of launching people back, or even bursting through solid walls. This is a subtrope of Artistic License - Physics and is related to Playing with Fire. Fire is commonly thought in fiction to be a "physical object" That is not the case. Fire is the process of rapid oxidation. The flame, or the visible part of the fire is the energy given off of the hot gases that make up a fire. Basically, it is visible heat. Since the fire a pyrokinetic user is using is not seen burning anything then the user is basically controlling "heat". Heat in itself cannot produce concussive force. What produces the concussive force is the rapid combustion of materials and the after product is the heat. Thus, although the fire has oxygen it does not have a fuel source so it shouldn't be burning in th first place. Unless, it is burning the other gases in the air. But, then that would cause an explosion. Contrast Video Game Flamethrowers Suck, which is when flamethrowers are portrayed as spraying pressurized gas rather than burning liquid; compare and contrast Hollywood Fire, which is another form of unrealistic fire--but instead of for the reasons above, it's due to the fire lacking proper heat and smoke.
ExamplesAnime and Manga