Warning: rampant speculation.

I think Urithiru is a giant spaceship. Yes, you've seen that before. But I think it was Urithiru crashing into the ground that created the Shattered Plains.

We've seen Radiants full to the brim of stormlight air dropping from the sky in Dalinar's visions, and with Kaladin at the end of WoR. When they land, a glyph appears on the ground around them and fades away.

Glyphs are symmetrical. We know also that the shattering in the shattered plains is symmetrical (with the exception of weathering and erosion in some areas).

We also now know that Urithiru has a gigantic "warp core" of gemstones.

I think Urithiru isn't the ship that first brought humans to Roshar, but perhaps a ship that someone created in an attempt to escape Roshar and Odium. Maybe that was a contingency plan of the KR - get enough people from Roshar onto Urithiru to be an ark of sorts and escape to a different planet. Maybe the 9 heralds who broke the oathpact wanted to escape their fate.

Either way, the ship tried to take off, but Odium put it back down. When it landed, the insane impact and ridiculous amount of investiture contained in the gem core printed a "glyph" on the plains on a huge scale, similar to when individual KR land on the ground. It shattered the plains in the symmetrical pattern that remains today.