Always hard to guess who leaked, but look to who gains.
Club :
1. If they want Baz to stay and see out his contract, they have no motive to leak news of his letter of demand. It simply destabilises the joint to the point where him remaining becomes untenable
2. If they want him to go, they have every motivation to have that occur via his own voluntary resignation or by his breach of contract (because in either of those cases they don't have to pay out his contract). Making his position untenable increases the pressure on Barrett to quit, or to accept a much reduced payout so he can make a clean quick break.
Barrett
1. If he wants to stay then leaking his letter of demand makes little sense, as it simply destabilises the joint to the point where him remaining becomes untenable
2. If he wants to get out, it helps him to have it known that the team's failures may in part be not all his own fault. But does it really help him for that to be public? Insiders in NRL clubland looking for a new coach could easily get all that info in assessing whether they want Baz. Leaking it may help him only in one way - by bringing it all to a head, thus speeding up a resolution with Manly so he can quickly become available for other jobs eg Panthers. But it is a messy way to do that and damages his own credibility as well. Besides which, the Panthers job seems a stretch at this point - a premiership contender signs a coach whose team is battling for the spoon? Seriously?
Ethically there is no doubt Barrett's manager could be behind the leak, but personally I doubt that is with Barrett's own knowledge or blessing. Doesn't really add up, as it harms his reputation, though it does potentially speed up his Manly exit
Ethically there is no doubt Manly (Penns/Zorba) could be behind the leak, as they've pulled this kind of low stunt in the past (with Toovey). It forces things to a head, makes Barrett's position untenable, and increases that chance that he won't get a payout or only a reduced payout.
So if I was a detective the club would be my chief suspect!!