Lilac dropped leaves

"Lilac Sunday" planted last fall as a one gallon plant. Good showing of flowers this spring, growing well in full sun/great drainage location in moderately (6.5) acid soil.

Periodical cicadas this year, plant was bagged in bird netting for four weeks. When I took the netting off last weekend I found a few mealybugs near the base of the plant where the bag had been tied. Rubbed most of them off then sprayed insecticidal soap. New growth apparent, all is well.
Checked daily and insecticidal soap sprayed as needed for a few more stray mealybugs. All told I have seen perhaps 20 mealybugs total- I check everywhere everyday.

This weekend we dug around a bit well outside what I would assume to be the root zone in order to install a border. I pulled up a bit of grass and weeds, but honestly- nothing was disturbed within at least a foot of the stem. New (shredded cypress) mulch laid down yesterday morning not touching the plant. By late evening a massive leaf drop had taken place. The remaining leaves will drop if you blow on them except for a few which seem fairly well attached. There are plenty of new leaves making an appearance and even some flower buds, but an alarming number of leaves fell.

No leaf discoloration at all- no chew holes, no mildew, no yellowing or chlorosis, no brown spots or areas. Just perfect green (although somewhat wilted appearing) leaves falling off.

Obviously we did something. We did have a massive thunderstorm yesterday which dropped nearly an inch right after I had watered the plant thoroughly, but aside from that and what I have described nothing has taken place that would IMO trigger a leaf drop like took place.

Ideas?

I wouldn't panic. It could just be plant shock.

Caused by?

(I have a need to understand ! :) )

A plant can be shocked anytime it is replanted. Sometimes they will loose some leaves or look droopy for a few days. It doesn't mean the plant is going to die or anything. I may just take a few days for it to adjust to it's new home.

It wasn't replanted- it's been in the same spot since last October.

My best guess would be that when you dug around it to add the border, you severed some feeder roots so the plant can't take up as much water. If this is the case, it will slowly recover. Try watering it with a little Super Thrive. This is not a fertilizer (which would burn the severed roots) but a mix of vitamins and hormones which are very beneficial to plants. When the plant has recovered, you will start seeing tiny new leaves develop along the stems.

Take a close look at the stems, especially around and near the base of your shrub. The worst pest of this species (especially on small young plants) is the lilac borer. If you have one of them tunneling around under the bark surface, you might lose the plant.

I doubt you caused any significant root disturbance to a plant that was just in a one gallon container last fall. In fact, that is a lot of stem for how tight the root system would've been restricted in a 1G pot. I hope you loosened up the root ball before you planted it, or it could be drying out a lot between rainfalls or watering. Alternatively, if you have poor drainage, coupled with above average rainfall/watering, your lilac may be swimming. Your pic doesn't seem to show that. If you can take some close ups of various parts (especially if you see anything odd), post them.

The only other change you made that might have an effect: fresh cypress mulch. I don't have any firsthand experience with that product, but any fresh mulch that could have decomposition still occurring may produce chemical byproducts that could give your plant a sudden jolt as they wash/leach into the soil.

Last thought: any mature walnut trees within spitting distance?

I see no evidence of any borers.
We always smush up the rootballs when we plant. We have very good drainage. There is a hickory tree (several) in the yard but no walnuts.

I thought about something in the mulch, but the leaf drop occured perhaps three hours after we put the mulch down- that's pretty fast for something leaching, no?

Today the remaining leaves have darkened considerably and drop at the touch. There is, however, one section where the leaves remain firmly attached and healthy looking, plus new leaves still seem fine all over.

Such a mystery!

I do have Super Thrive on hand and will use it the next time I water. I am uneasy applying any now as rainfall has been adequate to heavy the last two days and I don't want to be just throwing stuff at it.