They may want to sell, move in themselves, or do major renovations that require the place to be empty. Or they are bulldozing to build themselves. Or they don't like you. It can be any reason or no reason at all. They are incredibly difficult to fight.
I agree. We were given notice to vacate after 2.5 years, the only reason being the owners wanted vacant posession. The house never went on the market, even though they stripped it back & repainted. I'm assuming they already had a buyer lined up. They also refused to resign a lease for the last 2 years too. I believe its because THEY didn't want to be locked under a contract for any length of time, since they had considered selling before we moved in.