It's JKR's world, but everyone writes Potterfic eventually...
Young dementors have a fear of flight.
When full-grown dementors mate, they form their young by the mutual application of the Dementor's Kiss. (The irony of this is of course lost on those who had a vested interest.)
The new dementors then coalesce in a misty fug, hugging the ground because they dare not rise above. They possess no cloak, literally or metaphorically, in which to defy the sun. Perhaps the despair and hopelessness they spread reflects their own insecurities.
Then again, perhaps not. Only as free-floating adults do they induce fear- the rest of the time, they feel it.