They turned out to not be a good match for you, or a better match than you was available for them. At least the game thought so.
If it happens repeatedly, go to the home screen, press the button with lines on it (that used to be Start) while Halo is selected, and select Quit. Wait 5-10 seconds and restart it. If that doesn't work, force reset your Xbox One by holding the on/off
"button" for 8 seconds.
If it still doesn't work, give up, it's an intermittent server problem and will be fixed in an hour or so (only to break again the next hour).