It could be something as simple as the way they smell.
Once, while we were staying with my late father, Benji, the collie cross, i had at that time, suddenly flew from the livingroom and sat trembling in the kitchen. Nothing would coax him back in and only carrying him back solved the problem :
My son, Paul, had taken a bath using his grandad's unfamiliar bubble bath and it was Paul that Benji wanted to escape. he was fine, once Paul had got dressed and the scent become less obvious.
Barbara, Monty, Joly and Teddy.