[checks work e-mail]
Here's the message I just received-
"Your 710am United flight to Denver is delayed because we're looking for a new crew. Your original crew has not had sufficient off-duty/rest time required by federal law. We value your time and we're sorry for the inconvenience. UA866 now departs Raleigh/Durham at 746am and arrives at 925am"
Just say it's a mechanical issue.