Pretty common, and I beleive normal, condition. The older leaves die off and let the newer ones take over. If you just planted them outside recently, it could just be sun exposure. The older leaves are used to low light and artificial light. They have a tough time adjusting to the direct sunlight, where the newer leaves adjust much better. I wouldn't be concerned at this point....very common for bottom leaves to yellow and even die off. keep and eye on it though.
Bruce