Chase reducing your balance is likely their way of putting the capital they set aside as your credit limit to used somewhere else.

If the 5k they have set aside as your limit is not earning them interest due to you not using it,........then they saw it fit to reduce your limit and move the funds somewhere else where they could be productive.

The $hitty part about it is that it typically reduces your credit score because now you have less available credit to tap.

I'm not defending Chase. IMO they're a criminal organization considering the sordid history of JP Morgan/Chase Manhattan banking interests. I dropped my accounts from Chase when they bought out WaMu,........... If you're using them now I suggest you do the same.