Not sure. Possibly they're only walking a short distance between two places they'll need to wear a mask and feel there's no point taking it off to put it back on again? I must confess that there have been a couple of occasions when I've come out of a supermarket and walked for about 10 minutes before I've remembered that I have my mask on, so it's easy to forget (mind you, I work in a hospital and wear one for twelve hours so maybe easier for me to forget). I don't know why they're doing it but they're not harming me so I'm learning to ignore it...