Plunging pushes your coffee through the metal filter. Pushing harder pushes the coffee through the filter with a higher pressure. Higher pressure pushes particles past the filter more easily than lower pressure. Coffee particulate has a tendency to make coffee more bitter.
Anything that gets more small particles past your filter will add to your coffee's bitterness. This can happen when using a ground coffee that has a lot of fine particles, or if you agitate when you pour/plunge.
Anything that gets more small particles past your filter will add to your coffee's bitterness. This can happen when using a ground coffee that has a lot of fine particles, or if you agitate when you pour/plunge.