Character was just rolled to default league no apparent reason

You were killed while loading the next area. This put your chaaracter in default, but still in the process of loading a hardcore area - this caused the disconnection and 'out of league' message.
It takes time during the process of leaving an area for the server to remove you from the world, and you can be attacked during this time. It is advised not to use area transitions while monsters are present or could be nearby.

Sorry to hear about your character. While we're working on improving loading to reduce the time you're vulnerable, there'll always be some time while the server does it's thing.