Reference:
https://www.wolfsberg-principles.com/sites/default/files/wb/pdfs/faqs/17.%20Wolfsberg-Risk-Assessment-FAQ

Structuring transactions is a common method of money laundering, where cash deposits or withdrawals are broken down into smaller amounts to avoid reporting or record-keeping requirements. Employees who unintentionally assist customers in structuring transactions may not be aware of the legal and regulatory implications of their actions, or may have been misled or coerced by the customers. Therefore, the most appropriate and proportionate response is to provide remedial training to these employees, to ensure they understand the anti-money laundering policies and procedures, the red flags of suspicious activity, and their reporting obligations. Terminating, contacting law enforcement, or transferring these employees may be excessive, premature, or ineffective measures, depending on the circumstances and the level of involvement of the employees.
References:
ACAMS Study Guide for the CAMS Certification Examination - 6th Edition, Chapter 2: Money Laundering Risks and Methods, page 31.
ACAMS CAMS Certification Video Training Course, Module 2: Money Laundering Risks and Methods, Lesson 2.3: Structuring and Smurfing.

Explanation
Using cash to buy multiple cashier's checks over a period of time is considered suspicious activity of money laundering. This method is commonly known as structuring, where individuals or entities use cash to buy multiple monetary instruments such as cashier's checks in small amounts, to avoid reporting requirements.
This behavior could be an attempt to evade transaction monitoring and reporting requirements by a financial institution. Additionally, the fact that the account also receives multiple electronic transfers for the same amount could be a red flag for money laundering as it is a common technique used to layer or disguise the origin of illicit funds.
Reference: Certified Anti-Money Laundering Specialist (the 6th edition) Study Guide, page 100-101.