My personal theory, made from compiling all the informational tidbits we can collect, from rumours, choices of phrasing, little slips of the tongue in both characters and lore.
Basically the II Primarch spect too long in the warp, and came out absolutely f**ked. Mutated, corrupted, broken. He was raised on a desolate world, where he grew up to be an almost bestial creature, a shambling thing of horns and bulging muscle that prowled through the fields at night, eating cows and breaking machinery and all that lovely stuff.
Sone day the farmers are out tending to their labors, when suddenly the Legio Astartes descent from the sky on roaring pillars of fire, with HIM at the front. As soon as he hears of the beast, his mood grows somber, and he tells the assebled to hold as he "Takes care of some personal work"
Hours later, he returns, his armour and his hair flecked with Ichor, a single tear of sorrow on his visarge as he orders the forces off planet. Eventually the II legion is ground into dust by being deliberately placed in the hardest fighting of the crusade.
(Please forgive me for that little narrative)
The XI Is a different story. Rather, he went utterly insane and rebelled.
The XI Primarch also spends a lot of time in the warp, and arrives on a feral world. Despite this, like every other Primarch essentially, he puts together a great army and conquers the planet, turning it civilised.
On one day, a great golden breast descends on wings of flame, to alight atop the tallest mountain, directly before the Primarch, and his most favoured wards. The great maw opens, and issues forth a proud father with his own Custodians. The father and son embrace each other, whilst the wards try to look more menacing to the giant gold plated warriors that get a bit amused at the whole thing, in a polite way of course.
The Primarch goes home, is trained up, and is given command of the XIth Legion, and begins recruting from his home world, like all the others, including augmenting his favorite wards so they may continue to serve in his retinue.
All is going well until he finds himself and his legion caught in a collosal battle, that along with finally killing off the IInd, requiring the intervention of big E, also kills off most of his legion and his Wards, who basically raised him and were his life companions. After this, an assortment of gene defects show up both in Primarch, and legion, and many, including the primarch, begin to develop nearly every single kind of mental illness possible.
The now insane Primarch departs the great Crusade, his addled mind now believing, from what he saw, that the Emperor wishes to kill every last one of them all, by making them fight and die in such battles. He returns to his home world, razeing the population centers and begining to frantically convert new soldiers to replace his colossal losses, declaring himself apart from the Imperium of Man, and the new Warlord of humankind.
In responce to this, the Wolves are loosed. They fall upon the world, butchering through the untrained and inexperianced legion, and the insane veterans of the Crusade days with equal ferocity. Only a few of the old Wolves participating truly realise the sadness and the horror of their actions and what they mean. The Wolf King himself enters the throne hall of the castle alone, to confrunt his wayward brother. The Primarch of the XIth is sitting on the throne, mouth foaming and his uunkempt body twiching in paranoia and madness. He does not even notice Leman Russ, as he whispers his regret, and brings his sword blade down.
The legion is slaughtered to a man, the few remnants of a population are taken away and mind-wiped, all recors are scrubbed, and the Primarchs are forbade from speaking of them ever again, Leman internalises some deep sh*t, and becomes even more intollerant of the more petty issues faced by other Primarchs (Corvus for his obsessive Emoness and Lion, for being a prissy little MILKSOP) And damn I was trying not to write a story, but oh well.