actually, this isn't all that uncommon. Basically, the plants first grew a bunch of leaves used to the relatively cool and grey weather of early spring. When the hot sun came around, those leaves are ill suited to the sun and warmth. They are thinner "skinned" so dry out faster and may even have a different ratio of the different types of chlorophylls to suit colder weather. The plant reacts by replacing them with leaves more suited to our summer weather. Most plants do this to some extent all summer long, but with the drastic weather changes we have had lately, it's a little more apparent than usual.

I've noticed that many evergreens are dropping more needles than usual this year too. Part of the same general pattern, I think.