I can't say I have ever linked sticky figures being down to certain times of the year but I don't really take too much note of when I buy, usually in Winter though, as its boring and you can't spend your money on being outside! lol
I don't think its totally unlikely to be the heat, I have had figures "tacky" from sellers and in fact you tend to find its more the ones that have been MOC (have opened the odd beater here and there) that exhibit this stickyness, its probably a mixture of age and storage conditions.
Also - storing figures in certain ziplocks will make them tacky, the plastic doesn't react well with some bags so its best to be careful how you store them.
I wouldn't worry too much as its not anything that a good clean won't resolve, so just buy away regardless of the time of year and any funny figures will have to just have a dunk in some warm soapy water