You may have been attributing the assignment to something you observed that wasn't actually the cause; correlation≠causation. Mathematically the only reason for a call to be exercised early is as @FSU stated when there is a dividend or hard to borrow situation. Nothing to do whatsoever with anyone's suppositions about future price. If you were assigned early absent one of those reasons, congratulations, someone who didn't grasp options basics just gave you some free money!