This is new....and early.

It looks like they wanted to avoid resembling actual words in most cases beyond WAR for Warrior. That's how Paladin becomes PLD (avoids PAL, both pal and the PAL format), for example. And as pointed out, they seem to want to avoid ending vowels.

Let's see...
WARrior
MoNK (avoids MON, perhaps to avoid people calling them monkey or the Japanese word mon)
WHite Mage
BLack Mage
ReD Mage (instead of REM, like the band or the phase of sleep)
THieF (not sure, THI doesn't seem to be a Japanese word)
PaLaDin (as I said, avoids pal and PAL)
DaRk Knight (good one. DRK invokes dark/darkness. Only other possibilities that might ever come to mind are dork & dirk. Dar... isn't that the dude from Beastmaster?)
BeaSTmaster (avoids BEA, which is to avoid players from getting the status ailment Earworm. "Thank you for being a friend.... travelled down the road and back again...")
BaRD (yeah, they like to avoid vowels and words like BAR)
RaNGer (avoids RAN, though not Random Number Generator, not that many would get confused anyway)
SAMurai (only one that invokes a name, Sam. For some reason Square went straight with Samurai & Ninja, like Warrior)
NINja (likewise, seems they felt NIN invokes ninja. Well, it does. How many think of ninny or nincompoop?)
DRaGoon (yeah, looks like another pattern of avoiding ending vowels and having contractions that encapsulate the name. DRG does look like it invokes dragon)
SuMmoNer (avoids SUM. Hmm having a sum sub would be combining multiple math functions. All the game would need then is MUL, DIV, INT, DIF or DDX)
BLUe Mage (mage nomenclature would say BLM or BEM, though BEM looks kinda stupid and BLM was taken. Anyone not think of blue with BLU? Only one that ends in a vowel because what are their options really?)
CORsair (corsair is such an uncommon word. BUC for Buccaneer might've been more familiar in America. They didn't want Pirate for some reason, so decided to give the Caribbean Gambler the name Corsair)
PUPpetmaster (PUP invokes puppets. Only pet job too with the word pet in its name. And BST & PUP are the only masters. Seems DRG & SMN don't get much autonomy over their pets)
DaNCer (avoids DAN and situations where parties are made up of Dans & Sams hanging out)
SCHolar (they lucked out, 3 starting letters with no vowels and SCH makes one think of scholar, school, scholastic)