I most certainly can explain it! The whole idea of becoming a Slayer because of a great tragedy happening to the individual does not apply with Fyreslayers, and neither do they have the suicidal ‘glorious death’ viewpoint (only Doomseekers preserve this latter aspect of Slayer culture). Essentially you need to look at Fyreslayers as if they are classic Dwarfs but with the Slayer aesthetic. They all become Slayers because that is how they honour their warrior god - Warriors of Grimnir fight not by cowering behind armour, but instead by hurling themselves into every combat with axes swinging and warcries on their lips - but their culture more emulates that of the old Dwarf kingdoms in the world-that-was than the Slayer Cult, probably due to it being more sustainable (that’s really why their civilisation has survived - if they’d had retained the aim to consistently seek a glorious death, they would have probably died out a long time ago, because in Fantasy lore while Slayers were especially skilled, units and armies of them suffered huge numbers of casualties). That’s why I like them so much - they have the fun Celtic/Viking aesthetic of the Slayers, yet they preserve the culture of the old Dwarfs which is now pretty much dead in its original form given that Dispossessed have been shunted into the AoS equivalent of the Empire and Kharadron have largely abandoned their religion for science and Ferengi-esque business codes.