Unexpected

Question
Re. unexplained loss of Buff Orpington
My mixed chicken flock, hereto previously healthy, two weeks ago started "picking." Four out of 24 birds show signs of bare bottom spots. I have treated w/ a mixture of Desitin and GoldBond Med. powder, sometimes mixed w/ olive oil. One of the birds is a rooster. They continue w/ good appetite and drinking water.
New flock
New coop, 8' by 8', lino floor, good ventilaton, hanging water and food inside during winter, some food/water hanging during summer
Sliding door and ramp for coop access
Enclosed yard, 12-inch deep w/ clean sand, 25' by 35', fenced and wired perimeter and top.
Constant temp in coop 30F, 4 feet above floor. All roosts well spaced 4' or more above floor.
My two roosters fairly reasonable.
Two months ago, I started a 'treat' program w/ some cracked corn on ground outside in the mornings; regular feed and water are always in coop only. In summer, water also hanging in fenced yard. For winter, water is set on heating pan (not hanging) in coop.

Yesterday morning I found a dead Buff in a floor-level nest, head hanging over side of box. No exterior damage, no picking evident on this hen. Coop temp 32F. Rest of flock looks okay, eats/drinks normal; egg production same (but Buff may have been a current layer).
Possibilities:
Hen was stepped on inadertently by other hen in a low next box?
Disease made her weak to changing temperatures?
Coccidiosis?
I have removed next box from floor level.
Should I stop feeding cracked corn treat on ground outside? (It is not a raked clean area).

Apologies if I previously posted this in wrong place....
Thank you from Smith Hill.

 

Thank you, Penn