+1, I learned something new two weeks ago about the ATC reason - don't know if it's 100% true but I'll believe it.

What happens is if for some reason a plane goes MX or crew, etc., the delay is initially registered as MX, crew, etc, but then they try to schedule a new time for departure, and ATC may inform that the new time will not work and delays perhaps 5 minutes after the suggested time, so the new delay reason gets updated to ATC, even though it was MX that caused it to become ATC.