It is normal but it can happen in special cases.
Forex Strategy Builder checks the Required Margin at the end of every bar. If your current position is at a loss and the Free Margin is lower then the Account Equity FSB will raise a Margin Call. At that time it will close the current position.
There are two options after a Margin Call:
1. If the Account Balance is lower than the Required Margin for opening a new position, no more trades are possible. This is the most common case when you trade one lot. (No averaging);
2. The Account Balance is enough to cover the Required Margin. In this case the trade can continue. It can happen when the previous position's amount was greater than one lot. FSB closes the position because you haven't enough money to maintain these lots open, but you still have enough money to cover one lot.