Without a lot more detail, it's hard to know for sure.
One possibility is that you were giving a more detailed answer than the interviewer wanted to hear. Most times, an interviewer will have a set of questions that they want to go through and they only have a limited time to get through them. If a candidate spends too much time answering some questions, that leaves less time for the remaining questions. An interviewer may need to interrupt to keep the conversation moving forward.
I suppose it's also possible that the interviewer is acting rudely intentionally in order to see how a candidate reacts to rude customers. That's more likely if the position is something that has a lot of direct action with customers. I'm far from a fan of that sort of thing but I'm sure there are interviewers that take that approach.
Undoubtedly, though, there are other possibilities. Perhaps the interviewer was just in a bad frame of mind-- perhaps she had a major project due and was anxious to get back to her desk to finish it. Perhaps the interviewer was just a bad interviewer. Perhaps your communication styles just didn't mesh well-- it sometimes takes people time to figure out when someone else is pausing because they're done speaking rather than pausing as a natural part of speaking, for example. It's impossible to know with any real certainty what might have been in any particular interviewer's mind during any particular interview.