谁能帮我解释一下这道道德案例

(3) Eleanor Preston, the chief investment officer of Porter Williams Investments
(PWI), a medium-sized money management firm, has been trying to retain a difficult
client, Colby Company. Management at the disgruntled client, which accounts for
almost half of PWI’s revenues, recently told Preston that if the performance of its
account did not improve, it would find a new money manager. Shortly after this threat,
Preston purchases mortgage-backed securities (MBS) for several accounts, including
Colby’s. Preston is busy with a number of transactions that day, so she fails to allocate
the trades immediately or write up the trade tickets. A few days later, when Preston is
allocating trades, she notes that some of the MBS have significantly increased in price
and some have dropped. Preston decides to allocate the profitable trades to Colby and
spread the losing trades among several other PWI accounts.

这个案子，违反了3（B)。

不知道具体是如何违反了 fair dealing. 请给我一个解释。谢谢。