Let's keep in mind that after the legs touched the sloping lunar surface, SLIM was supposed to gently tilt/rotate towards the slope.
Perhaps, the rotation started too early and the spacecraft over-rotated. Maybe, one or more thrusters failed. The sideways rotation might have been caused by the spacecraft head jamming into the surface, thrusters still firing.
This is of course very coarse analysis based on what was displayed on the webcast. JAXA will figure it all out shortly.