There are two possible reasons for getting free loot boxes. First players with an Endorsement rating of 2 or higher will at random receive loot boxes as rewards. You will see a message that says “Keep it up!” with a picture of Roadhog giving a thumbs up if this is the case.
Also the other reason is that if you played in the first day of the version 1.26 (Wrecking Ball’s release) patch being released, there was a bug with the Loot Box system that caused new cosmetics from Wrecking Ball to not payout. As such all players who earned or purchased loot boxes during that time frame were compensated with the equal number of loot boxes at a later date. See this post for details: