it presents the most surface area to maximize aerodynamic braking.
The fins wiggle to adjust the drag in that corner to keep it in the right orientation (an angle of attack of ~70 degrees.
It's falling like a skydiver, not flying. Those are aerobraking flaps, not wings (they do not have an airfoil shape)
transitioning into the sideways fall, being stable in that fall, and transitioning quickly out of it to land were major achievements to check off.
for what it's worth, I think they did have engine problems. Musk said that they had low pressure for the landing burn, so that could account for the problems there. but it does seem odd that they would shut off the engines the way they did during assent, but the tweets indicate that it may have been planned. They did have the FAA limit their altitude more than planned, so they didn't need to get max performance out of the craft, just get it high enough to test the belly flop.
It will be very interesting to see how quickly they move on SN-9 (move it to the pad, pressure test it, mount engines, static fire, launch, probably at least a week but could be significantly longer)
It's also notable that these were Raptors in the 32-42 serial number range, and they did have some problems with them ahead of time. How much better are the newer Raptors going to be?