Does anyone know how the developers of Marlin decided to name it that?
As far as I know Erik van der Zalm started Marlin. He is from the Netherlands and Zalm translates to "salmon". One of the design goals of Marlin was to make it faster than the other firmware available at that time. And a marlin is a very fast swimming fish.
Some firmwares developed after Marlin also joined this fish theme: Sailfish, Minnow, ...
Yes, it probably is totally off-topic, but fun too, so I'll try to get an answer in, before the question gets closed.
The best place to ask would be the Firmware - Marlin forum, on RepRap.org.
The question is there now, Why was it named Marlin? and I'll update this answer, if/when I get a response...