Why might a vessel be more susceptible to capsizing in rough seas?

A vessel is more susceptible to capsizing in rough seas primarily due to the unpredictable and irregular nature of wave action, which can cause unintended heeling. In rough seas, waves can hit a vessel at various angles and intensities, leading to sudden shifts in the vessel's center of gravity and resulting in significant heeling. This uneven pressure from waves can push the vessel beyond its stable limits, making it more prone to capsizing.

While the weight and design of a vessel, as implied in some of the other options, certainly play a role in overall stability, the critical factor in this scenario is the environmental impact of rough seas on the vessel's stability. A vessel designed for calm conditions may not perform adequately under the stress of turbulent waters, and unless it is specifically built for extreme conditions, there is always a risk of capsizing due to the erratic movements induced by waves.

Rough seas do not increase the dining capacity of a vessel; instead, they challenge the vessel's stability, making this option irrelevant for the question. Thus, the irregular and unintended heeling created by rough seas is a direct cause of increased susceptibility to capsizing.