Thanks for your question.

It is hard to say what happened to your lilac. It sounded as if it was a rather quick occurrence and affected the branches at one point in time. I am struck by the fact that the leaves on the downed branches look rather healthy. None of this is typical of a lilac leaf borer infestation or a disease pathogen. But before looking at your pictures, I was inclined to think deer and/or rabbit action? But the branch breakage appears to be at their base. Rabbits and deer would leave some type of stub above ground. That still leaves the question as to cause? I am very positive that there is no threat to your surrounding plants. If I had to make a guess, and it is not even an educated one, is that squirrels might be the culprits. Do you have squirrels on your property?  For various reasons large numbers of grey squirrels have chosen to live in my yard. They are now working on their second (third?) litter of babies for the season. They are continually climbing in my lilacs to get green branches for their nests. From time to time their weight in the branches causes things to break. The branches on your lilac that broke looked to be old. You did say that the lilacs were mature. With age, lilac branches especially weaken at their bases.

Bottom line. Just remove these downed branches from the shrub. New growth will come up from the root. Don’t worry about adjacent plantings.

I really would appreciate hearing back from you regarding these thoughts.

Thanks for using our forum.

https://homeguides.sfgate.com/control-lilac-borers-21079.html

https://hortnews.extension.iastate.edu/lilacash-borer

Good luck.  Just keep an eye on things.  I suspect that this was a one-time occurrence.  However certainly feel free to get back to us if you see this again.

Thanks for using our forum.