I think it doesn't depend on the fact that you click "commence".
Most of the time I wait a few seconds before clicking it and it still happens from time to time.
Like yesterday, don't know what I queued for, but someone clicked withdraw once and I got kicked out of the queue completely.
Maybe it's not even someone withdrawing but someone getting disconnected, don't know.
I had hoped they fixed it with the last patch, but well ...

I posted a bug report a while ago: http://forum.square-enix.com/ffxiv/t...le-withdrawals