just ask them, page-2

Directors' resignation is usually a sign that they're fearful of incurring a liability for insolvent trading or being subject to other sorts of personal liability for breaches.
Problem is it's not that easy to establish. Not all directors are subject to personal liability even though to a larger or lesser extent they're liable. That's why a director of Lehman Bros got beaten up at the gym after the collapse. People take it into their own hands sometimes.