my lilacs have these curly stems that remind me of climbing vines. I have had these lilac bushes for years and have only noticed this phenomenon this year.
I have amended the soil with compost and mulched them about 2 weeks ago as they were previously surrounded by grass. The soil is very sandy in my area.
Since I have mulched them, I have noticed that there are multiple holes in the mulch beneath them where a critter has obviously been digging.
I am not familiar with Lilacs having curly stems rather than leaves, and since you have just noticed it, this structure may not be harming your shrub. It is too late to prune the shrub since the buds are set but next year after they bloom you might be able to prune the lilac to remove some of the curling features if this bothers you.
Removing grass in the area and mulching are positive actions for this shrub.
Reasons why stems do not grow straight may depend on many factors.
Lilac is regularly troubled by powdery mildews, but they do little damage to the plant in a typical year. Besides giving a lilac an unattractive appearance and allowing the fungus to spread to other plants, leaving powdery mildew alone when small colonies are on lilac causes little or no damage. Many types of powdery mildew only cause vague symptoms, including curled or distorted leaves, long before the characteristic white powdery fungal bodies appear, making early identification tricky. Longer-standing infestations may cause leaves to die and drop early.
Have you ever seen damage to the stems from insects? Look for holes in the trunk, and what might seem like sawdust on the ground. Hard to see with mulch. There is a lilac borer, Podosesia syringal, which is a larvae of a wasp like moth.
Recovery after a stem is damaged may make it grow differently. I am not sure of the growing conditions of your liliacs. Could the sun or wind be harder on this shrub this year? Was there ever damage to a branch? Do you prune all the same. Good air circulation?
Could the stress of irregular watering cause this curling? Water deeply and then only when the ground is dry again. We do get very hot weather which is hard on our gardens. The water needs of spring are not the water needs of hot summers.
Of course one might wonder if scientifically something genetically is going on with your lilac. Somewhere way back did it have a curly ancestor?
As Fall approaches, yes, it is coming, little creatures are finding nice homes. Mice? Voles? Watch for damage to lower stem. Maybe disrupt the mulch to discourage nesting.
Beautiful gardens take work!
Here is an excellent article on Lilacs for Cole Climates. This article discusses in detail site selection, care of lilacs as well as cultivar description.