When you disconnect, the server does not immediately take you out of the queue because it can take as much as a minute for it to time you out. Any time during that inactivity you can still be queued for a match and you are marked as a leaver, cancelling the match for the remaining players. This helps prevent queue-dodging.
Penalties are applied to all crashes and disconnections to encourage you to stop playing Competitive until you can resolve your issues. If you are disconnecting frequently, please start with these troubleshooting steps: