Being entombed into a machine that works as your contact with the rest of the world is a horribly traumatic experience. In the "Word Bearers" omnibus, we can find this fragment regarding Erebus dreadnought:
The meagre physical remnants of the inspirational leader had been interred within the
sarcophagus that lay before Marduk. Though his body was utterly ruined, he was destined to live on
within the tomb of his new shell, and become the Warmonger. While the other Dreadnoughts of the
Legion had slowly succumbed to madness and raving insanity, the Warmonger retained much of his
lucidity. It was his faith, Erebus himself had stated, that kept him from slipping into darkness.
So it seems that the whole process of becoming a dreadnought is just extending the inevitable madness and death - you need a really powerful mind driven by something like faith or vengeance to prevent yourself from slipping into a mindless monster. One obvious solution is to keep the ancient warrior asleep and wake them up only when the great need arises. Thus is the fate of the most famous loyalist Dreadnought - Bjorn the Fell Handed
Over the following five hundred standard years, he remained at the forefront of every battle, leading by example, and counselling his successors as the Chapter's Great Wolf. Eventually, the long years took their toll on the ancient warrior, and he began spending longer and longer periods dormant in stasis sleep. Now he is only awakened when the Chapter has the greatest need of his potent skills and ancient wisdom. [...]Bjorn is also awakened by his Chapter at the dawn of each new standard millennium to hold court at the Great Feast.
So from the fragment we can guess that the age and the stress of living in such conditions slowly starts taking toll on the mind of the entombed warrior.
Example of what happens to one entombed for too long, you can find in the "Red Tithe" novel:
‘Hail to you, Wandering Ancestor Itako,’ Nikora said, following the ritual
response to the awakening ceremony. ‘Arise, for there is blood in the water. Your
For a moment the Dreadnought didn’t respond. For a moment, the Chaplain
feared he had failed – Ancient Itako had once more slipped back into slumber,
his mind lost in the void of dream-memories he had spent so long swimming
Then there was a click, and a harsh blurt of static as the great Contemptor’s
external vox-units came online. The sound was followed by another, altogether
greater and more terrible.
With a howl of purest, frenzied rage, Ancient Itako heralded his own
Itako screamed. It was a horrible noise, part human, part machine, grating and
grinding from the vox-grille set in the war machine’s breastplate. It was
something inimical to the void brethren in the cathedra, a shattering of traditions,
a reminder that whatever was now nestled within the Contemptor’s armoured
shell, it was far removed from the warrior that had once been.
Such was the price of waking one of the Wandering Ancestors. Once unleashed,
there was little that could be predicted, bar that they would wreak devastation.