I used to work with a bank consulting company, and we would teach them whenever possible to roll-over expired deposits into much lower-paying accounts. The idea is to segment your market among price-sensitive and non-price-sensitive customers. So, the bank will run a promotional rate on something unusual, like a 7-month CD or something, and then drop the rates on its other accounts.
The customers that are paying attention will move their money into the promotional account for the better rate, while the lazy or otherwise unaware customers end up getting the rate dropped on them.