Without knowing more and examining the broken parts for clues, I'm just speculating about possibilities, rather than naming a cause.

A common cause of mysterious hanger breaks is something jamming in the chain and RD, so as you pedal, the jammed chain takes the RD back with it and snaps the hanger.

Another is a B screw adjusted improperly, allowing the jockey wheel riding too close, and engaging a sprocket through the chain in certain gear combinations. This can stress the TD and push it back with the turning of the cassette, and snapping the hanger.

There are others, but not being in low eliminates the most common cause, snagging a spoke.

You might try "reading the changer" which is usually ductile enough to show the direction of the break by way of the last bit bending rather than breaking clean. Once you know which way it snapped -- inward, backward, or at an angle -- that might give you clues to the sequence of events.