Was this rude for these people at work telling me to leave?

They were obviously having a private conversation but they were rude and out of line in asking you to move. It is unfortunate that you responded just as rudely to them as they were to you but understandable. Next time someone is rude to you try and take the high road. Had it been me I probably would have said "Excuse me for interrupting you but there is nowhere else to sit" and then I would have sat down. And when she would ask me if I minded sitting somewhere else I would simply have pointed out to her again that there was nowhere else to sit. Then I would have opened my book and started eating and I would have ignored any further comments on their part.