Not sure why he was doing steep turns, maybe a circling approach to the airport (if the weather wasn't good). Or maybe just a close-in visual pattern to land.
But, stall speed increases rapidly as bank angle steepens. If he was already slow, you get a stall on one wing, while the other wing is still generating lift. The result is a spin. Recovery takes 1500-2000 feet under the best of circumstances. If you aren't higher than that above the ground, it's over.
Stall-spin accidents happen most often on the turn from base to final (90 degree turn), because you are already low (500 feet AGL) and slowing to landing speed.
If he was making a visual approach, that would explain why he was making the turns: a standard pattern entry and landing downwind leg, base leg, and final approach (two 90 degree turns). If he makes a crosswind entry to the pattern, that adds one more 90 degree turn.
So can you explain why he would have 17 people on a plane with a capacity of 9? If there were children he may not have been over the weight limit but there’s still not enough seats.
Or he was trying to bleed off airspeed and altitude. That would put him ripe for a stall.