Pluph, since no one explained the details of the rules, I should remind you that the game will cancel at the start of a match when there is a leaver. Rejoining a game without penalty is only possible during certain phases of the game and within a timely manner. If you leave a game either during the pre-game or during the first minute of the game, the game will automatically cancel (remember the 10 second rule does not apply to the leaver, but as a warning to other players that the match will cancel). You can rejoin the match during the initial hero selection screen if you are lucky enough to quickly reconnect though.
Otherwise, you can only rejoin any-time after one-minute of match time has passed up until the match completes. However you only have two minutes to do so without penalty. If you rejoin a match after two-minutes, you will still automatically be marked with a loss an be penalized with a -50 SR cut and a ten-minute suspension. This rule ensures that no one can abusive the leaver system by attempting to return to a match after deliberately leaving in order to avoid such penalties. Furthermore leaving three or more times in a single match will result in an automatic penalty. If you fail to return to a match by the time it ends normally (regardless of match result) you will be penalized.
Unfortunately all disconnections and game crashes counts as leaving the match and will be penalized appropriately. The game server has no way to discern a legitimate disconnection or game crash from one that is forcefully caused by a dishonest player to bypass leaver penalties. You can learn about this policy here:
That being said, if you are disconnecting frequently, avoid competitive play. Remember if you see the message, “Lost Connection to the Server” in the title screen or the message “Failed to Connect to Server” in yellow text in the text chat box, this means there is something that disrupted your connection to the game server and not a problem with the game server itself. There can be a variety of reasons this happens including those that don’t disconnect your computer from the Internet. There is great troubleshooting to start with that can most likely resolve most issues and I encourage you to check them out here:
As always if nothing in that guide works, please reply saying you completed these steps and additional advice can be provided. Good luck!
(^^)v