Client setups different contact email addresses for each cPanel account. Why did the admin email receive Disk quota notifications?
Disk quota notifications are enabled by default to send emails to the admin or reseller of the accounts.
More details can also be found in the documentation link below.
WHM - Tweak Settings - Notifications tab
The settings to notify the admin can be disabled if desired. With these settings enabled, the contact address of the account will receive an email about their quota. The admin or reseller will receive a copy of this email.