I should explain that I am a game shooter, so only use my guns in autumn and winter.
Temperature alone does not promote rust, and I know a chap in the Highland region of Scotland who has recorded temperatures as low as -14 degrees C in his home one winter when his heating failed, and yet he has never experienced a rust problem.
However, he does regularly examine his guns at all times of the year because the real enemy is condensation. And that, I think, is the cause of your problem this year.
Do examine your guns regularly, and wipe the metal parts down with a lightly-oiled cotton cloth.
You can also create an anti-rust atmosphere in your cabinet with products available from your local gun shop.