A yield-rate environment in which long-term rates are decreasing at a rate faster than short-term rates. This causes the yield curve to flatten as the short-term and long-term rates start to converge. 

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When the yield curve is moving, it is either steepening or flattening. These fluctuations occur due to investor demand, changes in interest rates and institutional investors trading large blocks of fixed-income securities. 

If the yield curve is exhibiting bull flattener behavior, the spread between the long-term rate and the short-term rate is getting smaller because long-term rates are decreasing as short-term rates are increasing. This could occur as more investors choose long-term bonds relative to short-term bonds, which drives long-term bond prices up and reduces yields.