Bichon Friese Panic Attack?

Our 12-year old male Bichon has suddenly developed what I can best describe as "panic attacks." For no apparent reason, he will whimper, start shaking and demand attention. It's almost as if he is being frightened by something. He has always been very laid back. Our female, also 12, a puppy mill rescue, has always been hyper.
At times, it appears to be a bit of "separation anxiety," another situation which we've never had with him.

Causes?

GM