As others have suggested, we need more information about the context to be helpful in understanding the situation.

In general, though, sometimes people want to chat-- and sometimes people don't. Especially with acquaintances and coworkers (rather than close friends/family), if one is busy, stressed out, or on the move, they may just not want to stop and have a chat. If they have a limited amount of free time, they may want to use that time to spend it with family and friends and not just acquaintances who they happen to bump into. That doesn't make them rude or bad. I know that I personally don't really like to stop and chat when I bump into people. I'm usually pretty focused on what I'm doing or where I need to be next, and will often kind of duck out in order to avoid having a conversation. It doesn't mean I dislike that person, just that I'm not good enough friends to want to stop and chat and would prefer to get going with what I'm doing. Like, just today, when I ran into a neighbor, I hurried to my car and said "sorry; gotta go" because I was on my way to work. My neighbor is perfectly nice but she's a big chatter, and if I get into a conversation with her, it will last forever-- and I'm busy-- so I try to avoid it. There are also times I've been on the receiving end of those situations. Sometimes, when I bump into a co-worker at the office, I want to tell them a funny work story-- but they're busy doing a task and don't have time to hear the story. Not a big deal. When they're in the mood to chat another time, they will. At least at work, we tend to have a pretty good thing going where we all accept that sometimes, we want to be social, and other times, we want to just get our work done and can be honest about that with each other.