Maybe the person in question has had a bad experience in the past or just doesn't have good social skills and feels awkward at such events?
A friend of mine, who just started a new job recently and is still getting used to the new workplace, colleagues, etc., went to the work Xmas party last week and was told by a senior work colleague, who up until then had been nothing other than pleasant and helpful at work, that she wasn't up to the job and would never make it. Not only did he do that, he called over another senior work colleague and asked him to agree that she wasn't up to the job and wouldn't last!
Needless to say, she's devastated and is even more nervous going in to work than she was prior to the comments.