Old leaves yellow and die.
Another reason = too little fert, especially nitrogen.

Container-grown plants *must* be fertilized.

Jean is absolutely right.
Older leaves yellow and die, naturally...
but those leaves can be kept on the plant longer if you provide nutrients.

Aphids focus on the new growth, so you'll usually find them at the upper growth-tips.

Josh

I agree. I had one plant that has a tall stem and had several huge leaves on it. About three weeks ago, many of them yellowed and died and fell off. I wasn't worried since it was just the lower leaves and I figured it was pretty much normal. I have several other plants but they are bushier and dont have the long single stem. I dont recall a problem with them losing lower leaves at all. My guess it is more normal than a lack of nutrients but regardless, I wouldn't worry too much and controlled nutrients are a good thing anyway.