Been there, had that. I'd say about 50% of my cams come out somewhat concave on the sides, some with straight sides and one batch that looked like hamburger patties
I'm thinking it is probably more due to the wonderful dry climate we live in and how hard it is to keep them at the appropriate humidity. All the commercial geo/pc breeding rooms I've seen have fancy equipment to keep the RH at the right spot - for the whole life of the cheese up to wrapping. I'm trying to keep them in the mini-caves until ready to eat - even in the refrigerator part of aging.