The bow may lie in the nature of the formula of the specific batch of resin. Local temperature might have played a part, too, as you've noted. Not sure if you can ever be sure of the why. I've got one of those bases from Squadron, too, and it's flat as can be. It gets hot here in PA, too 
As far as preventing future bowing goes, I would secure the base to a wooden base underneath. I would drill some small holes and use small wood screws to secure it. They can be concealed easily enough with some putty to match the surrounding ground.
Hope that helps!
Best regards,
Brad