TAKING Money from Savings account WITHOUT PERMISSION

So FNB decides to deduct money directly from my savings account, without my knowledge. No idea what that money is being saved for but just because they see there is money, they assume it's absolutely acceptable to just shove their dirty hands in there and take without any explanation. Their Online bankers are of no help as they stay busy and you constantly have to see how you're nr 1666 in the queue. I need an explanation as to WHY my savings account is now short of almost R4k, money that is required for legal matters.