I've heard the barking dog on finals when positioning/pax in A319s on many occasions and pax often comment on it as they are getting off. I am uncertain of the logic behind the PTU activation, but one of its many uses is to maintain hydraulic pressure in both systems in the event of engine failure, so having it running during critical phases of flight is not a bad thing!
It is certainly not inhibited from running during flight (unless you have turned it off
) but it will activate if it senses a differential pump pressure of 500psi, so caused by extension of landing gear perhaps?