Trees, Shrubs and Conifers: Lilac dropped leaves, 1 by Cearbhaill

Cearbhaill wrote:
"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?