The phrase "troubling development" refers to a situation or event that is concerning or worrisome. It suggests that something has occurred or is happening that could potentially cause problems or complications. Full definition
Little else was known — and, after a long, troubled development cycle, little else still is. (theguardian.com)
But even those tiny victories for our characters are quickly overshadowed by more troubling developments. (mediamikes.com)
One of the most troubling developments has been the increase in adjustable rate mortgages that start out with a very low interest rate and then reset to a higher rate after a few years. (fhaloanpros.com)