Could have been a number of things as Auto cannons are still very common on the show. Re-watching a few bits and pieces on Renegade HPG channel, some parts are actual thought out (not many but a few) so someone may have noted a mech using indirect fire with other LRM equip mechs may have a better chance against Clanners vs getting into AC/20 rang. Could have also been a case of "So, there is no way we can use the Archer... we can use the Crusader ether... you know why. Look, here is a list of what we can use, pick whatever you think looks cool... yeah, but that's not a missile launcher on it's shoulder... I mean, I guess it could have a missile launcher... alright, you now have a Axman with missile for the show."