I'm afraid your photo isn't high-res enough for me to narrow down the list of possibilities.  I'll run through a few:

water - is it getting enough through these hot spells?
insects - an insect infestation could have come and gone before you looked for them.
herbicide drift - always a possibility on a plant growing next to a lawn.
disease - many lilacs suffered last summer from an unknown disease, read here:
https://extension.umn.edu/yard-and-garden-news/lilac-issues-season

This site might be of help:
https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/lilac/