Here goes nothing
Firstly, fluorine is not particularly abundant in the universe so you need a large source in order to get an atmosphere of it. Additionally, fluorine is easily consumed in stellar nucleosynthesis so it's hard to encounter a significant concentration for your planet forming. Anyway, let's begin!
A Star Dies
In a Red Giant star at the end of its life, it's inside is stratified with different layers of fusion occurring. In these different layers, you get different elements.Lighter in the outer layers, heavier in the center. The Star Dies in a core collapse supernova or Type II blasting its layers outward and releasing a ridiculous amount of neutrinos.
Deadly Neutrino Radiation
Relevant XKCD comic. Now the outflow of neutrino radiation is so great that it may be enough to knock a proton off Neon or Neutron to make Neon-19 which decays into fluorine. This process is currently speculation. The result is an expanding fluorine rich shell of gas. For the sake of argument, let's say there is a whole bunch of it. The whole system falls into a disk around the supernova remnant.
Planets start forming during your standard process: accretion. These planets form in the heavy element-rich layers with no abnormalities. Except two relatively large planetesimals form and collide with each other. These two planets were not so dissimilar from Earth during the early Haden Eon. So violent is the collision that the metal-rich cores of the planets are knocked out and form a new planetesimal rich in metal.
Our new planet drifts out into the fluorine rich part of the disk and begins gathering it due to its gravity. The planet has a surface rich in nickel and iron with much fewer silicates than earth. As the planet begins to gather fluorine the surface burns. But the metal rich surface reacts with the fluorine and produces a nonreactive passive layer. The silicates burn and react and form their own layers or there is not enough silicate to react with the fluorine. As such the planet is able to keep a fluorine atmosphere for a geological period of time.
A dense (why not venus atmospheric density for good measure) fluorine rich atmosphere with a whole bunch of other nasty compounds on its surface. Even More Hardcore it's also orbiting a neutron star or black hole.