Slack is reseting the passwords for all those users (approximately 1% of total) who hadn’t changed their passwords since 2015 when the company experienced a massive data breach. Hackers unauthorisedly gained access to one of the company’s databases that stored user profile information, including their usernames, email addresses, and hashed passwords. At that time, attackers secretly inserted code, probably on the login page, which allowed them to capture plaintext passwords entered by some Slack users. Slack has been sending a “password reset” notification email to all users who had not yet changed passwords for their Slack accounts since 2015.
Source: https://thehackernews.com/2019/07/slack-password-data-breach.html