It is quite normal for an occasional lower leaf on a mature plant to turn yellow and eventually fall. When several leaves turn yellow at the same time and then fall, the most likely cause is OVER WATERING or COLD DROUGHTS.

Leaves turn dull and lifeless

TOO MUCH LIGHT is the probably culprit
Another possibility is RED SPIDER MITE. Even healthy green leaves can be rendered dull and lifeless by dust and grime.

If edges remain green, the most likely cause is DRY AIR.
Another possible reason is BRUISING - People or pets touching the tips can be the culprit, so can leaf tips pressing against a wall or window.
If edges are yellow or brown, the possible causes are many and varied (OVER WATERING, UNDER WATERING, TOO LITTLE LIGHT, TOO MUCH SUN, TOO LITTLE HEAT, TOO MUCH HEAT, OVERFEEDING, DRY AIR or DROUGHTS. To pinpoint the cause, look for other symptoms.

Leaves turn yellow and fall

This is generally due to the use of CALCIUM in the compost of lime-hating plants or the use of HARD WATER for water such plants

If spots or patches are crisp and brown, UNDER WATERING is the most likely cause. If the areas are soft and dark brown, OVER WATERING is the probable reason.
If spots or patches are white or straw-colored, the trouble is due to WATERING WITH COLD WATER, WATER SPLASHES ON LEAVES, AEROSOL DAMAGE, TOO MUCH SUN or PEST/DISEASE DAMAGE.
If spots are moist and blister-like or dry and sunken, the cause is DISEASE.

spots or patches on leaves

brown tips or edges on leaves

Upper leaves firm but Yellow

wilting leaves

leaves curl and fall

What's Killing Your Plants

The most usual cause is either SOIL DRYNESS (caused by under watering) or WATER LOGGING (caused by impeded drainage or watering too frequently).
Another possible causes are TOO MUCH LIGHT (especially if wilting takes place regularly at midday)
TOO MUCH HEAT, POT-BOUND ROOTS, or PEST DAMAGE can also be a factor.

Curling followed by leaf fall is a sign of TOO LITTLE HEAT, OVER WATERING or COLD DROUGHTS.