Its not unheard of, and Honestly the same thing poped in my head about how the lower temps might have triggered it.
The reason they normally do not brumate young is because they are still growing, and need all of the food and time they can get to keep growing, and growing, and growing, so brumating is not triggered in them until they are their adult size.
I think thats what I read a long time ago anywaysI makes since though.