Corporate accounts are used to insulate the owners from operational losses (trading, bakery, bars...doesn't make a difference) and a corporate account without much, if any, track record will be asked for this guarantee, not just for trading, but for stuff like rents, credit lines, equipment purchases, etc. Unless you have significant credit history for your entity, a personal guarantee will be required for anything that relies on that credit history. (Yes, businesses have credit histories too. Instead of the big three reporting agencies, corporations/businesses have their own credit databases, such as DnB).
Since the potential for significant losses exist in a trading account, you can bet your ass they broker wants to make sure you are good for any losses incurred in your account.