Yes, it’s a trend, according to The Cassandra Report, which tracks millennial trends — in this case “gravitating toward…activities that provide both a sense of calm and a feeling of productivity…”:

COLOR MIND

Brands embrace the mindful coloring book trend
8 March, 2016

ART MINDFULNESS MARKETING PRESENCE OF MIND HOBBIES
Recognizing their tendency to become too immersed in their digital lives, young people are looking for ways to regain a sense of mindfulness and presence in the real world. They’re gravitating toward offline activities that provide both a sense of calm and a feeling of productivity; one such that’s gaining significant traction is coloring. Brands are jumping on this trend, creating coloring pages and books to address this growing need.