As I undertsand it (& I could be well off the mark) in "normal" conditions, the engines are pushing the boat & steering is controlled by changing the direction of the engine (thrust). So you actually contol the direction of the boat by moving the stern in the opposite direction that you want to go. In a following sea, the sea is pushing the boat from behind & depending on the relative speed of boat & sea, the sea could be pushing the boat faster than the engines are. At that point you have virtually lost control of the boats direction. The boat gathers speed due to being pushed from behind AND gravity as you slide down the face of the following wave & bonzai! you hit a small tree
As the bow digs in, it's pot luck which way the boat turns, depending on the angle of the waves & hull characteristics. It's a bit like trailer wag when going too fast downhill, the inexperienced brake too hard & the trailer passes you & you end up broadside on, on your side.
In a head sea, the opposite happens, it's the old immovable object & irresistible force scenario. The engines are pushing against the sea & the relative thrust hence steerage is higher. like dragging a trailer UP hill, much more stable & controlled. Planes have exactly the same issues with headwind & tail wind. Planes always take off & land into the wind if at all possible as the relative speeds & thus control, increases.