10 years is far too short.
I didn't know that about BRTs. I assumed they were like the big Airedales who can live to 19.
I agree with Kathy's suggestion to test a few more times leaving the house to see if the behavior is repeated. If it is senility or any age-related thing, I would think you would see other signs. Same if it were a health issue. Does he seem clingy when you are home? Is he sleeping more than usual? Any changes in appetite?
Maybe it has something to do with Razzle returning home and taking more of mommy's time? Magic had more of your attention while Razzle was away. Was the length of time greater than what it was in the past?