The designs are very deliberate, and were intended to be circular from the outset.
Bringing the language to the screen was a joint effort between designer Patrice Vermette, science consultant Stephen Wolfram -- of Wolfram Alpha fame -- and his son Christopher Wolfram. All told, some 100 "unique logograms with embedded words and phrases, with mutable components" were crafted for the film.
More than that,
Throughout a series of tweets recently, writer/producer Eric Heisserer explained not only how the circular speech symbols came to be, but also the "bespoke logogram analytic code" that translated the language when the cameras were rolling. "In several shots in the film, the analytics you see are working in real-time to dissect a logogram," Heisserer writes. "Not canned CG."
Also, they used linguistics professor Jessica Coon as a consultant.
A couple of interesting articles to read: