My lilac is in a container. Around July, for the first time, the leaves got brown and all dropped off. Then around August new leaves grew back
Plants such as lilacs when grown in a container will loose their leaves which will drop off if the container gets dry even for a few days. That it leafed out again later is a sign that drying up caused the leaf drop. If the plant is in a container for a long time it may be time to repot it into a larger container reducing the possibility that it will dry out easily in the future.