Question About Banks and Duration regarding, decrease, expected, increase

d2rockstar

On Page 27, Book 2 of Schweser, it says (regarding banks and duration), “If managers forecast increasing interest rates, they can decrease the duration of the assets by decreasing the duration of the securities portfolio. If interest rates are expected to decrease, they will increase the duration of the assets.”
Why will banks raise asset duration if interest rate decrease and lower asset duration if interest rates increase?
-Richard