I would say it is normal. As long as the plant look healthy otherwise, then don't sweat it. It happens more to some varieties than others but it is quite common. It seems more so on plants with bigger leaves. You might see some newer growth from that lower part of the stem as well.
Pictures are always welcome.
Bruce

Just "some", imo. Yellowing and dropping of lower, older leaves is completely normal. But if there's been an abundance in a short period, then it might be an environmental issue, such as high temps, drought conditions, or even mites.

In your case, with that raised bed. If you've been dealing with high temps for quite awhile, you have the perfect setup to rig up some shade cloth to try to regulate the temps of those blistering midwest days.

Kevin