A few examples of why: Banks/economies in melt down, less money available for loans to buy/refurbish that second some abroad, people perhaps less willing to part with cash to pay for weekends away in Carcassonokgraditz.... all adds up to uncertainty about future passenger numbers all adds up uncertainty about how much Ryanair will make in profit in the future, means uncertainty about the future share, means uncertainty about any dividends all leading to a weak share price, etc etc etc.....
... or Herc708 more neatly put it "the market"