First it was the engagement. Tanba revealed that he had gotten engaged with a woman from another ninja clan - Baren wasn't privy to if it was within or outside of Iga - for a continued strong blood-line. Baren could deal with that, really, he could, it wasn't like he cared about whatever feelings Tanba's finacée might have. Least of all if she wasn't from Oumi, which she might not be. But he could stick around for that, no problem. It was even alright when Tanba neglected him and spend so much of his free time with his future wife. Baren didn't care. No, that wasn't true, he cared, of course he did, but he understood. Sure he might throw tantrums about it and plan extravagant 'revenge plots' but he did understand. And Tanba said that he would only be this busy until the marriage. Only until everything was safely sorted out, because they were shinobi and shit could happen. And it wasn't that he minded being a so called 'mistress', not really.
When Iga had a skirmish at his friend Gengetsu's work in the Red Light District, he was reasonably upset, and confronted Tanba about it, one of his followers and subordinate members of the Momochi clan had told him to lay off and not get involved in things that was of no concern to him. Tanba had, after stopping Baren from drawing his sword, told his subordinate to step away, so him and Baren could have a private word. Baren glared daggers at him, but his wrist was still within Tanba's firm grip. Then Tanba explained that they had merely been seeking information, and that Kouga ninja had interrupted the discussion, Baren's glare was slightly subdued. He calmed down after he was told that Iga only fought to protect Gengetsu-dono, as she had potential information to impart to the clan, as well as her holding the position of Baren-dono's dear friend. After some grumbling - and a vicious punch that almost broke Tanba's nose - Baren forgave him.
But then Iga ninja outright assaulted the Cloud Shrine, and that was the last straw. Not only had they burned down half the shrine, but they had also harmed his sister, and that was a much greater offence. Baren had charged at Iga, into their forest and their village and had cursed them for harming the person that meant the most of all to him. Tanba made them stand down and only listen to the vicious insults, although it was clear that they were eager to retaliate to the verbal abuse. When Baren had finally left, it was not voluntarily, but rather, he was knocked over the head by his lover, and carried back to his home. Later, Tanba sent material for repairs of the shrine, sure, but that did not excuse his or his clans actions in any way. Because the shrine wasn't important to Baren; what was important was his twin sister, best friend, and other half.
Perhaps he could have forgiven the transgression if it had only been the shrine that was damaged, but by harming his sister, Tanba had crossed the final line, and there was no return.