Most of this info is coverted in Ep7-prequel book "Lost Stars".
The book offers explanation of the ship's end: The Rebellion tried to get the SD intact by boarding it; and sent Thane to disable the self-destruct.
Then the Inflictor shook with another impact. Another. A fourth. Each resulted in the same bizarre readings: gaps in the ship that had not resulted in vacuum. There could be only one explanation.
Ciena’s gut dropped. Although she’d never been aboard a ship when this had happened, she had learned the signs in the academy and relived them sometimes in her nightmares. “We’ve been boarded.”
Boarded. In the pitch of battle, that meant only one thing:
Her ship had to die.
And this is who prevented the self-destruct
“Get to the control center for engine three,” Thane ordered through his comlink as he edged down a corridor already thick with smoke. “If we can take out their last fully functioning main engine, we have a chance.”
Thane’s job was simpler and far more critical. He had to disconnect the self-destruct systems as soon as possible. Not one Imperial officer would hesitate before ordering the mass suicide necessary to keep a Star Destroyer out of New Republic hands.
When the captain tried the self-destruct, it didn't work:...
On my mark,” Ciena said. “Prepare for self-destruct. Initiating in ten—nine—eight—”
The Inflictor shuddered again. Even loathing the Empire as she did, Ciena was too much a captain not to feel a pang at the wounds to her ship.
She finished, “Three—two—one. Initiate.”
Ensign Perrin shoved down the lever that would set the self-destruct in motion. Ciena waited for the red lights, the siren, the automated announcement sending all crew to escape pods—her signal to seal the doors—but they never came. After the silence had lasted a moment too long, she raised herself from her chair to pull up ship schematics. Damage lights flashed in all the wrong places, in particular one area not far from the portside auxiliary bridge.
“They targeted the self-destruct systems,” Ciena said, almost in disbelief. “They specifically took them offline.”
So, she decided to crash the ship into the planet, to prevent it from ending up in Rebel hands:
“Awaiting your orders, Captain,” said a lieutenant standing in the data pits. She realized every person on the bridge—and probably throughout the ship—had no idea what to do next.
But she did.
The knowledge dawned inside her like the most beautiful day she’d ever seen. She could do her duty, fulfill her oath, and free herself from this madness forever.
“Don’t you ‘Commander Windrider’ me, not now. If the self-destruct were online, we’d have heard the automated signal. That tells me you’re planning on destroying the ship by—some other means—”
Ciena sat back down in her black leather chair, as weary as if she hadn’t slept in years. “Just say it.”
“…you’re going to crash the Inflictor into the planet.”
She began punching in the coordinates that would drive her straight into Jakku’s surface. Already she could imagine the fire, the heat, the end.
Then she would have done her duty to the last and yet escaped all the ties that bound her to the Empire, forever.
“I have to keep the Inflictor out of rebel hands no matter what.”
The book did not address why Inflictor did not disintegrate on impact
after Thane stunned Captain Cienna and punched out with her in an escape pod
But hey, it's an Imperial Star Destroyer. They are built strong :)