The likeliest explanation, which runs directly counter to the Muulsh Hypothesis, is simply that times were hard in Lankhmar, adventures few and uninviting, and that the two heroes had reached that point in life when hard-pressed men desire to admix even the rarest quests and pleasurings with certain prudent activities leading either to financial or to spiritual security, though seldom if ever to both.
This theory — that boredom and insecurity, and a difference of opinion as to how these dismal feelings might best be dealt with, chiefly underlay the estrangement of the twain… this theory may account for and perhaps even subsume the otherwise ridiculous suggestion that the two comrades fell out over the proper spelling of Fafhrd's name, the Mouser perversely favoring a simple Lankhmarian equivalent of “Faferd” while the name's owner insisted that only the original mouth-filling agglomeration of consonants could continue to satisfy his ear and eye and his semiliterate, barbarous sense of the fitness of things. Bored and insecure men will loose arrows at dust motes.
Certain it is that their friendship, though not utterly fractured, grew very cold and that their life-ways, though both continuing in Lankhmar, diverged remarkably.
Gray Mouser entered the service of one Pulg, a rising racketeer of small religions, a lord of Lankhmar's dark underworld who levied tribute from the priests of all godlets seeking to become gods — on pain of various unpleasant, disturbing and revolting things happening at future services of the defaulting godlet. If a priest didn't pay Pulg, his miracles were sure to misfire, his congregation and collection fall off sharply, and it was quite possible that a bruised skin and broken bones would be his lot.