My oldest daughter went through the same phase when she was around a year old. She went from being everyone's friend to being afraid of most men, especially those with beards. There wasn't an explanation, she just seemed to wake up one morning with fears that she didn't have the day before. She would cling to me or her daddy, and hide her face if a stranger got too close. It lasted for three or four months, then the fears just seemed to vanish as quickly as they had appeared. I think most children go through this phase. It could have something to do with suddenly becoming more aware of their surroundings, and going into some sort of sensory overload. They cling to the familiar until the new things aren't so new and scary. Hang in there. She'll probably grow out of it just like mine did.
Mine had a little bought of that when she was 1, but right around 2, she got it BAD. Like, someone would look at her and she would bury her face in my shoulder and not look up at all! This lasted, a month or so? And she is now seeming to get over it and enjoys attention from strangers again, lol.
My dd is like that at first with even people she knows! ...however after a good 20 minutes she turns into her normal self...so it's not sooo bad. She's been acting like this probably the past 6 months.