Technology

Is there a specific interaction that made them angry?

Stenzek's feeling got hurt when DuckStation was still proper open source software and people used the software fully in accordance with its license, i.e. they distributed modifications and not all permitted modifications were the most polished ones, so he felt that they give his name a bad reputation. Again: Stenzek released DuckStation under a license that explicitly allows this.

So he rage quit open source and released new DuckStation versions under a very restrictive "source available to look but not touch" license that's so insanely restrictive, Linux distributions are not allowed to make their own packages. So they ship the old version that works just fine because PlayStation 1 emulation was figured out very long ago. Stenzek feels that they should not ship the old version (which they are fully entitled to) and instead make a special exception for his software alone to point their users to DuckStation's website where instead of acquiring the emulator from their package manager (or "app store" in case you're not familiar with that term), Linux users should take extra steps to manually download and install DuckStation.

And since users may not know about this rift, they may post bug reports and feature ideas to Stenzek, even though these bugs may have been long fixed by non-open source DuckStation.

Basically: Stenzek did not read the license he picked for his software and then got mad when people made use of provisions explicitly allowed by the license.