The resets occur when the servers go down for maintenance such as when they went down today. I can confirm it happens on PC NA and I've seen other people say it happens on PC EU

As the maintenance is frequently occurring on Mondays this ends up wiping off the leaderboard shortly before the end of the week which means that people that don't play on Monday before the reset don't get credit and players that play a single match on a Monday with maintenance will generally hit the leaderboard even with a score of 0.