How many times did this happen in the lobby during the countdown? And did it happen any other seperate times?
Assuming that you are using the Wiimote it's possible that it might be an issue with your Nunchuk. The "Z" button is used to access the ally list in the lobby and to ADS during a match (assuming that this is your setup).
I would test using either a new Nunchuk, new Wiimote or both, if you can to rule this out. I've had this kind of issue before with other games due to a damaged "Z" button.
The closest I have had to this kind of issue and it not be a controller problem is in the campaign. The game would glitch out and I would be stuck running in circles as though my batteries had died. My controller was fine and the whole system wouldn't respond. I had to resort to holding down the power button on the console. Happened 3 times, two of which were in the same place of the same level.
Is the screen being shaky more due to gun recoil that seems more excessive than normal? I only ask this because I've had times where I know I'm being screwed by weird lag and my gun would be kicking like a mule whenever I fired - far more than it normally does.
If it is more because of the screen being more erratic while moving the Wiimote then it may be another indicator of a faulty controller.
I've got some pretty good ideas as to why lobbies are getting worse like this. I could be wrong though, since the quality of the connection in overall seems to have gotten worse. But people who shouldn't be hosting are pulling host all thanks to a glitch. It's safe to say that we're not getting a patch, even though I reported this months ago.
I also find it highly amusing that I read a tweet sent to @ATVIAssist from an XBOX 360 player, just a week or so ago, who is reporting the very same glitch that I reported and provided proof of months ago.
They will likely get a patch for it.