More than likely you had an open order, a withdraw or a reversal of coins (with LTC it probably was not the third) that was making your balance negative. In most of these cases you simply had to cancel. In your specific case I am not sure which it was but yes we do not let anyone withdraw if they have a negative balance.
I don't have any open order that time. The support said it was from a trade I did long time ago. And I had BTC trades after that.
The negative balance only appeared when I tried to withdraw. I still have my craptsy points in my account that I could sell that time so my account balance isn't negative. And you made me wait for hours like an idiot trying to figure out what went wrong. I didn't even receive any warning or an email.