[I]In what case do creditors go after your bank account? Is it the fairly large debt owed or any?

Depends on the creditor, the bank, the amount etc. If it's a very small debt it may not be worth the cost of collecting it. But as long as the cost of collecting is less then the amount that can be recovered they will probably go ahead.

I will agree with Scott. Each company will have its own rules, but often if they are going to get a judgement and they already know your bank account info from your credit reports ( banks now run reports normally when you open accounts) of even from their records if you paid them by check.

As a land lord I have often done it for 500.00 even though it cost to do it, I would have rather paid the para legal for their time than let the person cheat me out of the money

Creditors can go after your bank account once they've received a judgement against you authorizing them to do so.