Okay, so portentous means pompous or self-important, right?
Well, in that case, what's that got to do with portents?
Thing Not To Be Today: portentous. The first meaning of portentous in my Collins dictionary is of momentous or ominous significance, which is very clearly something to do with portents (portents are omens). The next meaning is, basically, awe-inspiring or miraculous, and again you can see the connection with portents. The third meaning is self-important or pompous - and there I lose the thread a bit.
Still, prophets aren't exactly known for their laid-back charm and ready wit, are they?
So the word portentous probably serves them right.