Why not print "back side down"? So you need specific line orientation?

The supports are only lightly connected to the object, to make them easier to remove. The force being exerted by the plastic as it cools is stronger than the light connection to the supports, especially over such a large area.

Probably there is a draft causing it to cool to rapidly. An enclosure, even just a cardboard box around the print area, would help.

Support gap too wide, causing model to lift off the supports- Or the bottom and sides warped up, causing them to bow inward and buckle that thin section.

I agree with other commenters, this seems like turning it 90 so the back face could sit flat on the build plate would likely have a lot better results. If that is not an option, lower the support Z-gap and increase the support interface layer density/decrease line spacing. Make sure your filament is dry. Decrease your cooling fan speeds.

Excessive cooling maybe? Or the support gap is too tall?

Edit: might also be nozzle temp too low

Not sure why it would warp there, perhaps infill percentage is too high?

If you don't need the layer lines to be in a specific orientation I'd consider rotating the model back 90° for the reprint, hopefully that should avoid any warping.

The heat from printing the end of those supports/ limbs. You need to reduce the heat as much as possible without clogging your nozzle.

Edit: A lot of people figured that out before me. Good luck!

Is there any reason you can't make the model in two separate pieces instead, with the floor and sides as one print, and the roof as another print, to either be glued or clipped together?

I'd think that would avoid the warping problem altogether, plus save wasted filament. Though in all fairness, it would still be nice to learn what's causing the warping anyways, for sake of future prints..