Your cat is sooo happy! No one knows for sure why some cats display this behavior, but is appears that they get so wrapped up in the euphoria of being gently stroked that they forget to swallow. Other signs of happiness a cat may show when they are being petted is loud purring, eyes half closed, and “kneading” the surface they are standing on. Some will even seemingly fall over on their sides, close their eyes, knead the air, drool and purr all at the same time. I have one client who told me that when her cat gets like this, he would “quick as lightening” reach out and grab her if she tries to stop petting him, so she has to very carefully disengage from her cat when he gets really happy from her petting.