While Brawl matches have several rounds (i.e. all dragons got to do an action), they do have a set end to prevent them from extending for too long. A Draw is when there is no clear winner, and no Trophies are won or lost. In order for this to trigger, you and your opponent must have the same amount of dragons left standing by the time the match ends. To summarize:

• Win: All opponent dragons are defeated or the end of the match is reached and you have more dragons left standing.
• Draw: The end of the match is reached and both you and your opponent have the same amount of dragons left standing.
• Loss: The opponent has defeated all your dragons or the end of the match is reached and you have less dragons than your opponent left standing.