Resolved Question

Could be a huge variety of reasons - we probably need more information. Is it all the leaves, or just the older leaves? As palms grow, the older leaves will brown and fall off, only to be replaced by new ones.

If the whole plant is going brown: it could be lack of water, or too much water, or that the plant has got root-bound in a pot which is too small for it. It could also be exposure to extreme cold or extreme sun. It could be disease or pests. Pretty much every major problem will cause the plant to brown off and die.

For reference, palms need full to partial sun (but not necessarily full sun if you live in a very hot climate), plenty of water, but they should be planted in a very free-draining soil. They hate to sit with their feet wet for too long - they will rot from the roots up. (Palms naturally live in very sandy soils). They generally hate frosts.

If these are indoor plants: every indoor palm that I've ever seen struggling, it was due to lack of sunlight, lack of feeding and lack of water. They need to be watered a couple of times a week - but any water that remains in the saucer an hour after watering, should be thrown away. Feed them regularly with a liquid fertilizer in their water - at least a couple of times a year. :)

I hope that helps...