A radiographic film becomes too dark and fogged, due to:
FILM FOG

Improper safelighting (improper filter; excessive bulb wattage; inadequate distance between safelight and work surface; prolonged exposure to safelight)
Light leaks (cracked safelight filter; light from doors, vents, or other sources)
Overdevelopment 
Contaminated solutions
Deteriorated film (stored at high temperature; stored at high humidity; exposed to radiation; outdated)

White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition
Inappropriate processing
Concentrated developer
Poor film storage
Longer exposure duration