Normally one wants a bit of weather helm. If the wind increases suddenly the boat will head up and stall. It is "in irons" or the more current and pathetic term in the "no go zone." There are two inherent problems which require reducing sail as quickly as possible :
1. Once the boat is in irons there is no forward motion. It becomes at the mercy of the unmerciful waves. And it falls back off only to be overpowered and head up again. And again. This is not good.
2. When running downwind the impact of the boat heading up is that the apparent wind (the wind the boat sees, the vector sum of the boat speed and direction and the wind speed and direction) actually gets stronger. Not good. Since the easiest way to deal with a strong wind is to run away this adds complexity. Usually the autopilot can not compensate if the sail plan is that out of balance. Hence the advice: "reef early, reef often."
Fair winds and following seas :)