The reasons for most stink vent pipe flashing leaks are because improperly flashed or have just aged with the weather. The rubber boot at the top base of the aluminum or plastic flashing usually gets old and cracks that's one reason why it'll leak. The second reason improperly flashTheed not properly far enough up under the shingles and what happens is it leaks through the top through the water lines of the roof. Or a lot of lazy people will just go put a new pipe flash covering over the old one and I think that'll do it or they might just tire it and then overseas and it leaks again. If you feel that your stack pipe might be leaking around the flashing call professional that knows what they're doing they get it looked at to get it properly repaired.