In today's economic climate, delinquencies are rapidly increasing. This explains why credit lines are being reduced. Also, cards are being closed down. It is belt tightening time for banks as well as us.

It sounds like you carry balances on your cards. Making "payments on time" to me means paying the card off every month. This "minimum payment" trickery they came up with a long time ago I don't understand. They may have targeted you as a higher credit risk than others, or maybe they just clotheslined everyone. I have an Amex here, and I haven't had any changes made to it or my other cards.

The bonus of having your credit line reduced is that you won't be able to carry a higher balance and have to pay interest on it.