When there's a disagreement in open source projects, there's the potential for a fork. One set of developers disagrees with the direction of a project and creates a (often permanently) divergent version. This is the hard fork – and any open source project can be forked.
'Getting forked' is as bad as it sounds. Developers, users, and all other sorts of resources are split. New users might be confused which project is the canonical project.
The forked distribution might offer up a feature that disrupts your price-setting power – imagine a forked ad-free Google Chrome (Brave?). Or after an acquisition by a bad steward (projects forked after getting acquired by Oracle: LibreOffice, Jenkins, MariaDB). Or by a company with a completely different business model (etc., Ads vs. Software, Software vs. Hardware).
There have been multi-billion dollar companies that started off as forks from projects. The original founders of the forked project often get nothing.
Here's a timeline of interesting forks.
2003 – Wordpress is forked from b2/cafelog. The b2 developer, Michel Valdrighi, stopped updating the software, prompting Matt Mullenweg to create a fork called Wordpress. See his blog post.
2006 – Oracle Enterprise Linux forked from Red Hat Enterprise Linux. Oracle no longer had to pay Red Hat.
2009 – The creator of MySQL forked the project as MariaDB after it was eventually acquired by Oracle (via Sun). MariaDB just raised $104mm in a Series D, expecting to go public next year.
2013 – Google forks a new project (Blink) from Apple's WebKit. Blink is the engine that powers Chrome, WebKit powers Safari. WebKit was originally a fork of KHTML.
2017 – Bitcoin Cash (BCH) hard forked from Bitcoin Core (BTC). Bitcoin Cash wanted to increase transaction throughput and increased the block size to 32 MB. BCH has a market cap of $7 billion, BTC has a market cap of $888 billion.
2017 – PrestoSQL is forked from PrestoDB. The PrestoDB project was originally created at Facebook in 2013. Three of the original creators of Presto left Facebook in 2017 to start a company called Starburst Data. They forked PrestoDB to create PrestoSQL.
2019 – Amazon forked OpenJDK after Oracle started charging for Java licenses for business users.
2019 – Amazon forked Elasticsearch ("Open Distro for Elasticsearch") after Elastic changed the license of their Elasticsearch project from Apache v2 to Server Side Public License (SSPL), which prohibited Amazon from selling a managed version of the project.