What was the reason for the roar of laughter that echoed as Leacock left the bank in "My Financial Career"?

The roar of laughter can be attributed to Leacock's actions while attempting to make a deposit. Leacock's excessively furtive behavior and fumbling actions likely made him a comical figure in the eyes of the bank employees.
In the story, Leacock tries to deposit fifty-six dollars at the bank. His overwhelming challenge, however, lies in the fact that he has little trust in banking institutions. His nervousness initially lends an air of mystery to his person. The bank manager thinks he is a detective or perhaps even a very wealthy man who doesn't want anyone to know the amount of his deposit.
When the manager discovers that Leacock just wants to deposit fifty-six dollars, his interest in Leacock wanes. He asks the accountant to help Leacock instead.
The story ends on a comical but sad note. Leacock wants to deposit fifty-six dollars and draw out six dollars for his present use. However, due to his nervousness, he writes the full deposit sum of fifty-six dollars on the withdrawal slip. To spare himself the embarrassment of admitting his mistake, he blusters through the entire incident and insists that he will withdraw the entire amount he just deposited.
This is why a roar of laughter erupted after Leacock left the bank.