This is another question I get asked a lot, I can only answer from my own findings. Firstly older leaves usually turn yellow and fall off this happens around the winter months and should not be a concern. Secondly it could be caused by too much light this also causes yellowing of the leaves, and too little light will make the leaves turn a darker Green and elongated, the ideal would be a nice light Green in colour.Thirdly over watering can also cause yellowing of the leaves, but usually the leaf’s drop before they turn yellow and fourthly, check your roots you may well have a problem with root rot usually caused by being too wet for to long remember they need to be moist at all times and not saturated so review your watering frequency, and finally do you have good air circulation, fans etc. You never said where you grow your Masdevallias?