Unlikely, unless you have no drainage holes in the pots, or they're left sitting in water filled trays all the time. Leaves on Phormiums do die back and fall, a few at a time, going brown first, so this could just be normal leaf loss. Excessive wind can make the leaves tatty and may cause some browning, and the other thing to check for is Phormium mealybug - to do this, you will need to look right at the base of the leaves, inbetween them where they're tightly crammed together, for white, fluffy deposits.

Umm, Bamboo? Is this Phormium or Cordyline?

Oh dear, sorry Guest - there I am wittering on about Phormium when you're asking about Cordyline - answer's basically the same, the lower leaves do die off, so if its the lower leaves only, its probably just natural aging. Forget the mealybug thing though, that only applies to Phormiums.

If it's not just the older leaves, and there are dark brown patches and spots on the leaves, that might be overwatering. Light brown tips is usually underwatering, low humidity, or salt damage from bad water. If there is a yellow zone between the green part of the leaf and the brown part, that makes it more likely to be salt damage.